[llvm] [AMDGPU] Teach SIPreEmitPeephole pass to preserve MachineLoopInfo (PR #178868)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

Better to actually update the loop info if available. This conditional preserve only helps the new pm 

https://github.com/llvm/llvm-project/pull/178868