[PATCH] Teach InstCombine to simplify (A & ~B) in icmp

[David Majnemer]

The attached patch can transform:

(A & ~B) == 0 --> (A & B) != 0
(A & ~B) != 0 --> (A & B) == 0

if we can prove that A is definitely a power of 2.

Last time I tried this (almost exactly four months ago), self hosted clang
builds failed because of undefined behavior. Somewhere.

I've tested this patch against a checkout that was compiled by a compiler
that was compiled with this patch and it went fine.

Tests that exercise both transforms are included.

-- 
David Majnemer
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