[llvm] [WIP] Implemented a patch to optimize SGPR spills. (PR #93509)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Implemented a patch

Title should describe what this is, not that it's a patch

> In order to facilitate this optimization, needed to preserve some additional analysis results throughout both phases of SGPR and VGPR register allocation {within StackSlotColoring pass}.
This should be a separate PR 

> Also, while achieving so, resolved a BUG in StackSlotColoring pass related to SlotIndex updation.

This should also be a separate PR 



https://github.com/llvm/llvm-project/pull/93509