GlobalISel: Artifact combine merge-like and unmerges into merge-like

GlobalISel: Artifact combine merge-like and unmerges into merge-like

Recognize when sub-vectors have been split to elements which are used to
build large vector.
This happens when instructions have different vector sizes available.
For example a few arithmetic instruction are required to process all
elements of larger vector that can be stored using one instruction.

Differential Revision: https://reviews.llvm.org/D109242