[llvm] [StackSlotColoring] Fix issue where colors for a StackID are dropped (PR #138140)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

> What if you have pre-existing frame indexes, and then run through the allocator?

I don't think those are included. IIUC it's only the `InlineSpiller` that updates the LiveStackAnalysis:
https://github.com/llvm/llvm-project/blob/a7aca819d44b8d67f2cffd452e6b63741c83cd62/llvm/lib/CodeGen/InlineSpiller.cpp#L1260-L1264 I can't see any reference to `getOrCreateInterval` anywhere else in the codebase and the InlineSpiller appears to create the stack slots (in `assignVirt2StackSlot()` just before). 

https://github.com/llvm/llvm-project/pull/138140