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

Summary:
We already do it for matching splat constants, but not just values.

Further improvements for non-matching splat constants, as noted in
https://reviews.llvm.org/D46760#1123713 will be needed,
but i'd prefer to do that as a follow-up.

https://bugs.llvm.org/show_bug.cgi?id=37603
https://rise4fun.com/Alive/cplX
https://rise4fun.com/Alive/0HF

Reviewers: spatel, craig.topper

Reviewed By: spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D47981

llvm-svn: 334372