[llvm] [AMDGPU] Stop reserving $vcc_hi in wave32 mode (PR #87783)

[Matt Arsenault via llvm-commits]

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

I've also been wondering what happens if you unreserve exec_hi. Are writes discarded? can you use it as a free SGPR?

https://github.com/llvm/llvm-project/pull/87783