[RISCV] Add test for experimental.vector.reverse

```
void vector_reverse_i64(int *A, int *B, int n) {
  #pragma clang loop vectorize_width(4, scalable)
  for (int i = n-1; i >= 0; i--)
    A[i] = B[i] + 1;
}
```
When option: scalable-vectorization is on (or set #pragma clang loop vectorize_width(elements, scalable)), Reverse Iterators can't loop vectorization as <vscale x elements x elementType>

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D125866