Perl_sv_setsv_flags: handle mixed IV and NV fast case

When the fast code at the start of Perl_sv_setsv_flags was modified to
also support bodyless NVs, the simplest possible change was made.
However, this meant that there was no fast handling when one SV was an
IV and the other a NV. Actually having this seems desirable since it
avoids the need to allocate (and later release) an XPVIV or XPVNV body.

Author: richardleach

sv.c

@@ -4201,7 +4201,7 @@ Perl_sv_setsv_flags(pTHX_ SV *dsv, SV* ssv, const I32 flags)
     STATIC_ASSERT_STMT(SVt_IV   == 1);
     STATIC_ASSERT_STMT(SVt_NV   == 2);
 #if NVSIZE <= IVSIZE
-    if (both_type <= 2) {
+    if ((stype <= SVt_NV) & (dtype <= SVt_NV)) {
 #else
     if (both_type <= 1) {
 #endif
@@ -4276,6 +4276,10 @@ Perl_sv_setsv_flags(pTHX_ SV *dsv, SV* ssv, const I32 flags)
         return;
     }
 
+#if NVSIZE <= IVSIZE
+    both_type = (stype | dtype);
+#endif
+
     if (UNLIKELY(both_type == SVTYPEMASK)) {
         if (SvIS_FREED(dsv)) {
             Perl_croak(aTHX_ "panic: attempt to copy value %" SVf