AVX code generation improvements

1. Allow fallback to a lesser AVX support during make

Due to the fact that some distro restrict the compiule architecture
during make (while not setting any restrictions during configure) we
need to detect the target architecture also during make in order to
restrict the code we generate.

2. Add comments and better protect the arch specific code.

Identify all the vectorial functions used and clasify them according to
the neccesary hardware capabilities.
Use these requirements to protect the code for load and stores (the rest
of the code being automatically generated it is more difficult to
protect).

3. Correctly check for AVX* support.

Signed-off-by: George Bosilca <bosilca@icl.utk.edu>

Author: bosilca

ompi/mca/op/avx/configure.m4

@@ -44,7 +44,7 @@ AC_DEFUN([MCA_ompi_op_avx_CONFIG],[
            #
            # Check for AVX512 support
            #
-           AC_CACHE_CHECK([if we are checking for AVX512 support], op_avx_check_avx512, AS_VAR_SET(op_avx_check_avx512, yes))
+           AC_CACHE_CHECK([for AVX512 support], op_avx_check_avx512, AS_VAR_SET(op_avx_check_avx512, yes))
            AS_IF([test "$op_avx_check_avx512" = "yes"],
                  [AC_MSG_CHECKING([for AVX512 support (no additional flags)])
                   AC_LINK_IFELSE(
@@ -115,14 +115,14 @@ AC_DEFUN([MCA_ompi_op_avx_CONFIG],[
            #
            # Check support for AVX2
            #
-           AC_CACHE_CHECK([if we are checking for AVX2 support], op_avx_check_avx2, AS_VAR_SET(op_avx_check_avx2, yes))
+           AC_CACHE_CHECK([for AVX2 support], op_avx_check_avx2, AS_VAR_SET(op_avx_check_avx2, yes))
            AS_IF([test "$op_avx_check_avx2" = "yes"],
                  [AC_MSG_CHECKING([for AVX2 support (no additional flags)])
                   AC_LINK_IFELSE(
                       [AC_LANG_PROGRAM([[#include <immintrin.h>]],
                               [[
-    __m256 vA, vB;
-    _mm256_add_ps(vA, vB)
+    __m256i vA, vB, vC;
+    vC = _mm256_and_si256(vA, vB)
                               ]])],
                       [op_avx2_support=1
                        AC_MSG_RESULT([yes])],
@@ -134,8 +134,8 @@ AC_DEFUN([MCA_ompi_op_avx_CONFIG],[
                        AC_LINK_IFELSE(
                            [AC_LANG_PROGRAM([[#include <immintrin.h>]],
                                    [[
-    __m256 vA, vB;
-    _mm256_add_ps(vA, vB)
+    __m256i vA, vB, vC;
+    vC = _mm256_and_si256(vA, vB)
                                    ]])],
                            [op_avx2_support=1
                             MCA_BUILD_OP_AVX2_FLAGS="-mavx2"
@@ -164,29 +164,29 @@ AC_DEFUN([MCA_ompi_op_avx_CONFIG],[
                          CFLAGS="$op_avx_cflags_save"
                         ])])
            #
-           # What about early AVX support. The rest of the logic is slightly different as
+           # What about early AVX support? The rest of the logic is slightly different as
            # we need to include some of the SSE4.1 and SSE3 instructions. So, we first check
            # if we can compile AVX code without a flag, then we validate that we have support
            # for the SSE4.1 and SSE3 instructions we need. If not, we check for the usage of
            # the AVX flag, and then recheck if we have support for the SSE4.1 and SSE3
            # instructions.
            #
-           AC_CACHE_CHECK([if we are checking for AVX support], op_avx_check_avx, AS_VAR_SET(op_avx_check_avx, yes))
+           AC_CACHE_CHECK([for AVX support], op_avx_check_avx, AS_VAR_SET(op_avx_check_avx, yes))
            AS_IF([test "$op_avx_check_avx" = "yes"],
                  [AC_MSG_CHECKING([for AVX support (no additional flags)])
                   AC_LINK_IFELSE(
                       [AC_LANG_PROGRAM([[#include <immintrin.h>]],
                               [[
-    __m128 vA, vB;
-    _mm_add_ps(vA, vB)
+    __m256 vA, vB, vC;
+    vC = _mm256_add_ps(vA, vB)
                               ]])],
                       [op_avx_support=1
                        AC_MSG_RESULT([yes])],
                       [AC_MSG_RESULT([no])])])
            #
            # Check for SSE4.1 support
            #
-           AC_CACHE_CHECK([if we are checking for SSE4.1 support], op_avx_check_sse41, AS_VAR_SET(op_avx_check_sse41, yes))
+           AC_CACHE_CHECK([for SSE4.1 support], op_avx_check_sse41, AS_VAR_SET(op_avx_check_sse41, yes))
            AS_IF([test $op_avx_support -eq 1 && test "$op_avx_check_sse41" = "yes"],
                  [AC_MSG_CHECKING([for SSE4.1 support])
                   AC_LINK_IFELSE(
@@ -202,7 +202,7 @@ AC_DEFUN([MCA_ompi_op_avx_CONFIG],[
            #
            # Check for SSE3 support
            #
-           AC_CACHE_CHECK([if we are checking for SSE3 support], op_avx_check_sse3, AS_VAR_SET(op_avx_check_sse3, yes))
+           AC_CACHE_CHECK([for SSE3 support], op_avx_check_sse3, AS_VAR_SET(op_avx_check_sse3, yes))
            AS_IF([test $op_avx_support -eq 1 && test "$op_avx_check_sse3" = "yes"],
                  [AC_MSG_CHECKING([for SSE3 support])
                   AC_LINK_IFELSE(
@@ -224,8 +224,8 @@ AC_DEFUN([MCA_ompi_op_avx_CONFIG],[
                          AC_LINK_IFELSE(
                              [AC_LANG_PROGRAM([[#include <immintrin.h>]],
                                    [[
-    __m128 vA, vB;
-    _mm_add_ps(vA, vB)
+    __m256 vA, vB, vC;
+    vC = _mm256_add_ps(vA, vB)
                             ]])],
                              [op_avx_support=1
                               MCA_BUILD_OP_AVX_FLAGS="-mavx"