Reland AMDGPU/GlobalISel: Combine zext(trunc x) to x after RegBankSelect

Recommit bf5a5826. Depends on
4c8fb7dd which was reverted.

RegBankSelect creates zext and trunc when it selects banks for uniform i1.
Add zext_trunc_fold from generic combiner to post RegBankSelect combiner.

Differential Revision: https://reviews.llvm.org/D95432