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

[Sander de Smalen via llvm-commits]

https://github.com/sdesmalen-arm commented:

Would it be possible to create an MIR test that does a `run-pass=stackslot-coloring`, using objects with `scalable-vector (2)` and `noalloc (255)` stack IDs to expose this case? I don't think this pass cares specifically about the meaning of these stack IDs.

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