[InstCombine] 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.

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

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: llvm-commits

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

llvm-svn: 334371