[AMDGPU] Recognize x & ((1 << y) - 1) pattern.

Summary:
As a followup for D48007.

Since we already handle `x << (bitwidth - y) >> (bitwidth - y)` pattern,
which does not have ub for both the edge cases (`y == 0`, `y == bitwidth`),
i think also handling a pattern that is ub for `y == bitwidth` should be fine.

Reviewers: nhaehnle, bogner, tstellar, arsenm

Reviewed By: arsenm

Subscribers: arsenm, kzhuravl, wdng, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Tags: #amdgpu

Differential Revision: https://reviews.llvm.org/D48010

llvm-svn: 334816