[GISel] Rework trunc/shl combine in a generic trunc/shift combine

This combine only handled left shifts, but now it can handle right shifts as well. It handles right shifts conservatively and only truncates them to the size returned by TLI.

AMDGPU benefits from always lowering shifts to 32 bits for instance, but AArch64 would rather keep them at 64 bits.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D136319