[llvm] RegAllocGreedy: Fix use after free during last chance recoloring (PR #120697)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> LGTM.
> 
> Out of curiosity does AMDGPU rely on last chance coloring to work?

I encounter it semi-regularly. There are many issues related to subrange splitting (the most pressing may be that spill/restore loses live lanes). I think we need a new form of subrange split too 

https://github.com/llvm/llvm-project/pull/120697