[llvm] [AMDGPU] Fix simplification of VM_VSRC based on VMEM waits (PR #178711)

[Krzysztof Drewniak via llvm-commits]

https://github.com/krzysz00 approved this pull request.

Nitpickery, and please update commit message

LGTM modulo comments (and I'm curious what this Vsrc thing is, but that's not important)

https://github.com/llvm/llvm-project/pull/178711