[StackColoring] Don't merge slots with differing StackIDs

The documentation for this specifically mentions that this should not
happen. We could think about adding target hooks to permit it (and how
to merge IDs) in the future if that is desirable.

This specific test case was merging a scalable-vector slot into a
non-scalable one and dropping the notion of scalability, meaning we
failed to allocate enough stack space for the object.

Reviewed By: arsenm, MaskRay, sdesmalen

Differential Revision: https://reviews.llvm.org/D125699