[llvm] [AMDGPU] Don't optimize agpr phis if the operand doesn't have subreg use (PR #91267)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Patch description isn't clear. Instead of just showing an invalidly generated case, should show the case that was converted into the invalid case 

https://github.com/llvm/llvm-project/pull/91267