[llvm] [AMDGPU] Remove dead code in SIISelLowering (NFC) (PR #108198)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

I'd prefer to leave this as-is. The comment is still significant and these paths will deviate in a patch I'm working on 

https://github.com/llvm/llvm-project/pull/108198