[mlir][spirv] Add a pass to unify aliased resource variables

In SPIR-V, resources are represented as global variables that
are bound to certain descriptor. SPIR-V requires those global
variables to be declared as aliased if multiple ones are bound
to the same slot. Such aliased decorations can cause issues
for transcompilers like SPIRV-Cross when converting to source
shading languages like MSL.

So this commit adds a pass to perform analysis of aliased
resources and see if we can unify them into one.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D119872