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

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

The cleanup commits look reasonable; please submit them separately.

The IR representation of lifetimes has some pretty significant issues (see https://github.com/llvm/llvm-project/issues/45725#issuecomment-1537597333 , #51838, #132085, #43674).  Will this cause those issues to be exposed more frequently?

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