[InstCombine][NFC] dropRedundantMaskingOfLeftShiftInput(): change how we deal with mask

Summary:
Currently, we pre-check whether we need to produce a mask or not.
This involves some rather magical constants.
I'd like to extend this fold to also handle the situation
when there's also a `trunc` before outer shift.
That will require another set of magical constants.
It's ugly.

Instead, we can just compute the mask, and check
whether mask is a pass-through (all-ones) or not.
This way we don't need to have any magical numbers.

This change is NFC other than the fact that we now compute
the mask and then check if we need (and can!) apply it.

Reviewers: spatel

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68470

llvm-svn: 373961