[llvm] [AMDGPU] Fix nondeterminism in SIFixSGPRCopies (PR #70644)

[Joe Nash via llvm-commits]

https://github.com/Sisyph commented:

Can you add a test? I assume it would be a flaky test before this patch, but should pass every time after this patch.

https://llvm.org/docs/ProgrammersManual.html#llvm-adt-mapvector-h
The usage manual recommends removing elements in bulk, since with this container removal is slower. Can you extract the removal at L1043 V2SCopies.erase(C.ID) 
to a removeList and do it a after the loop?

https://github.com/llvm/llvm-project/pull/70644