[llvm] [AMDGPU][NFC] Pre-commit test for spilling a read-undef IMPLICIT_DEF (PR #227858)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Does this mean you're intending to special case spills of IMPLICIT_DEF? I don't think that's necessarily valid.

CodeGen as a whole doesn't properly handle poison vs. undef, and the status quo is approximately that IMPLICIT_DEF is poison, but undef is also selected to IMPLICIT_DEF 

https://github.com/llvm/llvm-project/pull/227858