[llvm] [AMDGPU][GlobalISel] Allow selecting buffer_load/store of pointers (PR #110714)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> 2. Vectors of pointers need to be bitcast to vectors of the corresponding scalars, since there doesn't seem to be a good way to define the rewrite patterns for buffer_load/store of those types

I'd hope you can declare them with VTVec and PtrValueType, but it probably requires new GlobalISelEmitter support. VTVec wants the enum value from MVT for some reason 

https://github.com/llvm/llvm-project/pull/110714