[AMDGPU] Skip debug instruction uses while optimizing live range of a reg in...

[AMDGPU] Skip debug instruction uses while optimizing live range of a reg in SIOptimizeVGPRLiveRange

This will prevent the `assert(!O.readsReg())` from firing in
SIOptimizeVGPRLiveRange::optimizeLiveRange

Fix for #64163

Reviewed By: arsenm, #amdgpu

Differential Revision: https://reviews.llvm.org/D156893