[InstCombine] Shift amount reassociation in bittest (PR42399)

[InstCombine] Shift amount reassociation in bittest (PR42399)

Summary:
Given pattern:
`icmp eq/ne (and ((x shift Q), (y oppositeshift K))), 0`
we should move shifts to the same hand of 'and', i.e. rewrite as
`icmp eq/ne (and (x shift (Q+K)), y), 0`  iff `(Q+K) u< bitwidth(x)`

It might be tempting to not restrict this to situations where we know
we'd fold two shifts together, but i'm not sure what rules should there be
to avoid endless combine loops.

We pick the same shift that was originally used to shift the variable we picked to shift:
https://rise4fun.com/Alive/6x1v

Should fix [[ https://bugs.llvm.org/show_bug.cgi?id=42399 | PR42399]].

Reviewers: spatel, nikic, RKSimon

Reviewed By: spatel

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63829

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@364791 91177308-0d34-0410-b5e6-96231b3b80d8