[llvm] [AMDGPU] Don't fix the scavenge slot at offset 0 (PR #79136)

[Matt Arsenault via llvm-commits]

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

I suspect we have some missing emergency scavenging slot test coverage, but in principle this should be OK 

https://github.com/llvm/llvm-project/pull/79136