[llvm] [regalloc][LiveRegMatrix][AMDGPU] Fix LiveInterval dangling pointers in LiveRegMatrix. (PR #168556)

[Scott Linder via llvm-commits]

https://github.com/slinder1 commented:

I'm not too familiar with the code, so excuse any misunderstanding, but is the bug in `LiveRangeEdit::eliminateDeadDef` or `SIPreAllocateWWMRegs.cpp`?

In the summary the focus is on `LiveRangeEdit::eliminateDeadDef` leaving dangling intervals, but it seems like that issue would still be present there after this patch?

https://github.com/llvm/llvm-project/pull/168556