[mlir][spirv] Add target control to UnifyAliasedResourcePass

The UnifyAliasedResourcePass is actually only necessary for
targeting Apple GPUs via MoltenVK, where we need to translate
SPIR-V into MSL. The translation has limitations--no support
of aliased resources. So introducing a control to disable
this pass if targeting other platforms.

Reviewed By: kuhar

Differential Revision: https://reviews.llvm.org/D136869