[GlobalISel] Fix invalid combine of unmerge(merge) with intermediate cast

Summary:
The combine for unmerge(cast(merge)) is only valid for vectors, but was
missing a corresponding check. Add a check that the operands are vectors
to avoid an invalid combine.

Without this check, the combiner would emit incorrect code for scalars
and pointers because the artifact cast (trunc/ext) only affects bits at
the end of the type, while this combine assumes that the casted bits
appear between meaningful bits.

This also uncovered a segmentation fault in the AMDGPU
InstructionSelector. The tests triggering this bug have been moved to
their own file and a check for the segmentation fault has been added.

Reviewers: arsenm, dsanders, aemerson, paquette, aditya_nandakumar

Reviewed By: arsenm

Subscribers: tpr, jvesely, wdng, nhaehnle, rovka, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78191