[llvm] [AMDGPU][CodeGen] Preserve debug value locations across multi-stage register allocation (PR #187237)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> `VirtRegMap` (`VRM`) have been lost due to the intervening passes such as `StackSlotColoring` clearing `VRM` between stages.

Can we fix those passes to preserve it? They shouldn't need to worry about maintaining it in any real way 

https://github.com/llvm/llvm-project/pull/187237