[llvm] [AMDGPU] Prevent genration of unused SGPR IMPLICIT_DEF assignments (PR #155241)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Not sure why there are so many end to end test changes. These are always deleted at the start of RA, unless they are live out of a block 

https://github.com/llvm/llvm-project/pull/155241