[GlobalISel] Combine sext([sz]ext) -> [sz]ext, zext(zext) -> zext

Summary:
Combine sext(zext x) to (zext x) since the sign-bit is 0
after the zero-extension.

Combine sext(sext x) to (sext x) and ext(zext x) to (zext x)
since the intermediate step is not needed.

Reviewers: arsenm, volkan, aemerson, aditya_nandakumar

Reviewed By: arsenm

Subscribers: jvesely, wdng, nhaehnle, rovka, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77210