[GlobalISel] LegalizationArtifactCombiner: Combine aext([asz]ext x) -> [asz]ext x

Summary:
Replace `aext([asz]ext x)` with `aext/sext/zext x` in order to
reduce the number of instructions generated to clean up some
legalization artifacts.

Reviewers: aditya_nandakumar, dsanders, aemerson, bogner

Reviewed By: aemerson

Subscribers: rovka, kristof.beyls, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D54174

llvm-svn: 347893