[llvm] [AMDGPU] Avoid put implicit_def into bundle that break reg's liveness (PR #142563)

[Jay Foad via llvm-commits]

https://github.com/jayfoad commented:

Just adding some comments for posterity, mostly for my own understanding. The description of this PR was never updated to explain what it does.

https://github.com/llvm/llvm-project/pull/142563