[flang] Update the description of !fir.coordinate_of

This change was suggested in one of the comments for
https://reviews.llvm.org/D115333. Basically, the following usage is
valid, but the current wording suggests otherwise:
```
%1 = fir.coordinate_of %a, %k : (!fir.ref<!fir.array<10 x 10 x i32>>, index) -> !fir.ref<!fir.array<10 x i32>>
```
A test is also added to better document this particular case.

Differential revision: https://reviews.llvm.org/D115929

Author: banach-space

flang/include/flang/Optimizer/Dialect/FIROps.td

@@ -1633,7 +1633,7 @@ def fir_CoordinateOp : fir_Op<"coordinate_of", [NoSideEffect]> {
     Compute the internal coordinate address starting from a boxed value or
     unboxed memory reference. Returns a memory reference. When computing the
     coordinate of an array element, the rank of the array must be known and
-    the number of indexing expressions must equal the rank of the array.
+    the number of indexing expressions must not exceed the rank of the array.
 
     This operation will apply the access map from a boxed value implicitly.
 

flang/test/Fir/convert-to-llvm.fir

@@ -2471,11 +2471,11 @@ func @coordinate_array_known_size_1d(%arg0: !fir.ref<!fir.array<10 x i32>>, %arg
 
 // -----
 
-func @coordinate_array_known_size_2d(%arg0: !fir.ref<!fir.array<10 x 10 x i32>>, %arg1 : index, %arg2 : index) {
+func @coordinate_array_known_size_2d_get_i32(%arg0: !fir.ref<!fir.array<10 x 10 x i32>>, %arg1 : index, %arg2 : index) {
    %q = fir.coordinate_of %arg0, %arg1, %arg2 : (!fir.ref<!fir.array<10 x 10 x i32>>, index, index) -> !fir.ref<i32>
    return
 }
-// CHECK-LABEL:   llvm.func @coordinate_array_known_size_2d(
+// CHECK-LABEL:   llvm.func @coordinate_array_known_size_2d_get_i32(
 // CHECK-SAME:    %[[VAL_0:.*]]: !llvm.ptr<array<10 x array<10 x i32>>>,
 // CHECK-SAME:    %[[VAL_1:.*]]: i64,
 // CHECK-SAME:    %[[VAL_2:.*]]: i64) {
@@ -2486,6 +2486,20 @@ func @coordinate_array_known_size_2d(%arg0: !fir.ref<!fir.array<10 x 10 x i32>>,
 
 // -----
 
+func @coordinate_array_known_size_2d_get_array(%arg0: !fir.ref<!fir.array<10 x 10 x i32>>, %arg1 : index) {
+   %q = fir.coordinate_of %arg0, %arg1 : (!fir.ref<!fir.array<10 x 10 x i32>>, index) -> !fir.ref<!fir.array<10 x i32>>
+   return
+}
+// CHECK-LABEL:   llvm.func @coordinate_array_known_size_2d_get_array(
+// CHECK-SAME:    %[[VAL_0:.*]]: !llvm.ptr<array<10 x array<10 x i32>>>,
+// CHECK-SAME:    %[[VAL_1:.*]]: i64) {
+// CHECK:           %[[VAL_2:.*]] = llvm.mlir.constant(0 : i64) : i64
+// CHECK:           %[[VAL_3:.*]] = llvm.getelementptr %[[VAL_0]][%[[VAL_2]], %[[VAL_1]]] : (!llvm.ptr<array<10 x array<10 x i32>>>, i64, i64) -> !llvm.ptr<array<10 x i32>>
+// CHECK:           llvm.return
+// CHECK:         }
+
+// -----
+
 // 5.2. `fir.derived`
 func @coordinate_ref_derived(%arg0: !fir.ref<!fir.type<dervied_4{field_1:i32, field_2:i32}>>) {
   %idx = fir.field_index field_2, !fir.type<dervied_4{field_1:i32, field_2:i32}>