[InstCombine] Shift amount reassociation in bittest: trunc-of-shl (PR42399)

Summary:
This is continuation of D63829 / https://bugs.llvm.org/show_bug.cgi?id=42399

I thought naive pattern would solve my issue, but nope, it involved truncation,
thus more folds needed.. This isn't really the fold i'm interested in,
i need trunc-of-lshr, but i'we decided to start with `shl` because it's simpler.

In this case, no extra legality checks are needed:
https://rise4fun.com/Alive/CAb

We should be careful about not increasing instruction count,
since we need to produce `zext` because `and` is done in wider type.

Reviewers: spatel, nikic, xbolva00

Reviewed By: spatel

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66057

llvm-svn: 369117