[llvm] [AMDGPU][RegAlloc] Correct insertion of IMPLICIT_DEF in loop headers (PR #186348)

[Janek van Oirschot via llvm-commits]

JanekvO wrote:

Title suggests you're changing regalloc but this is a spill lowering pass change, might need to remove the RegAlloc tag in title (unless there's a reason I'm missing as to why it should be there?)

https://github.com/llvm/llvm-project/pull/186348