GlobalISel: Artifact combine merge-like and unmerge into unmerge

Recognize when source could have been unmerged to pieces with DstTy
without having to split source to smaller elements
and then merge small elements into DstTy pieces.
This happens when vector was meant to be split to sub-vectors but there
was leftover. At this point artifact combiner have already dealt with
leftover and we can continue to use sub-vectors.

Differential Revision: https://reviews.llvm.org/D109241