[GlobalISel] fix widenScalarUnmerge if widen type is not a multiple of destination type

Fix creation of illegal unmerge when widen was requested to a type which
is not a multiple of the destination type. E.g. when trying to widen
an s48 unmerge to s64 the existing code would create an illegal unmerge
from s64 to s48.

Instead, create further unmerges to a GCD type, then use this to remerge
these intermediate results to the actual destinations.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D88422