[AArch64][GlobalISel] Combiner for EXT

Keep components of UNMERGE larger after running the Artifact Combiner on it.
This was intended to help with <v16i64> = G_SEXT <v16i16>, but implementation
for legalizing EXT is in a following patch, therefore a test for this case
will be included in the following patch

Differential Revision: https://reviews.llvm.org/D157715