[llvm] AMDGPU: Make VarIndex a WeakVH in AMDGPUPromoteAlloca (PR #188662)

[Shilei Tian via llvm-commits]

https://github.com/shiltian approved this pull request.

This is a cleaner cut. If the other approach is not needed, can you revert it in a follow-up?

https://github.com/llvm/llvm-project/pull/188662