[llvm] [AMDGPU][GlobalISel] Fix load/store of pointer vectors, buffer.*.pN (PR #110714)

[Petr Penzin via llvm-commits]

ppenzin wrote:

Local failures also with the patch reapplied, as well as that builder is still failing. It is quite odd, not sure what is causing that.

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