[AMDGPU][FIX] Proper load-store-vectorizer result with opaque pointers

The original code relied on the fact that we needed a bitcast
instruction (for non constant base objects). With opaque pointers there
might not be a bitcast. Always check if reordering is required instead.

Fixes: https://github.com/llvm/llvm-project/issues/54896

Differential Revision: https://reviews.llvm.org/D123694