[GlobalISel] ComputeKnownBits - use common KnownBits shift handling (PR44526)

Convert GISelKnownBits.computeKnownBitsImpl shift handling to use the common KnownBits implementations, which makes use of the known leading/trailing bits for shifted values in cases where we don't know the shift amount value, as detailed in https://blog.regehr.org/archives/1709

Differential Revision: https://reviews.llvm.org/D90527