[InstCombine] Recommit: Fold (x << y) >> y -> x & (-1 >> y)

Summary:
We already do it for splat constants, but not just values.
Also, undef cases are mostly non-functional.

The original commit was reverted because
it broke tests for amdgpu backend, which i didn't check.
Now, the backed was updated to recognize these new
patterns, so we are good.

https://bugs.llvm.org/show_bug.cgi?id=37603
https://rise4fun.com/Alive/cplX

Reviewers: spatel, craig.topper, mareko, bogner, rampitec, nhaehnle, arsenm

Reviewed By: spatel, rampitec, nhaehnle

Subscribers: wdng, nhaehnle, llvm-commits

Differential Revision: https://reviews.llvm.org/D47980

llvm-svn: 334818