[llvm] [StackColoring] Change the StackColoring logic + enables it to handle spills (PR #143800)

[Than McIntosh via llvm-commits]

thanm wrote:

I am willing to take a look, but for various reasons I won't be able to dig in for a couple of months (I am away from my work computers).  Hope you can make progress without my eyeballs for now. Thanks.

https://github.com/llvm/llvm-project/pull/143800