[InstCombine] match De Morgan's Law hidden by zext ops (PR22723)

This is a fix for PR22723:
https://llvm.org/bugs/show_bug.cgi?id=22723

My first attempt at this was to change what I thought was the root problem:

xor (zext i1 X to i32), 1 --> zext (xor i1 X, true) to i32

...but we create the opposite pattern in InstCombiner::visitZExt(), so infinite loop!

My next idea was to fix the matchIfNot() implementation in PatternMatch, but that would
mean potentially returning a different size for the match than what was input. I think
this would require all users of m_Not to check the size of the returned match, so I 
abandoned that idea.

I settled on just fixing the exact case presented in the PR. This patch does allow the
2 functions in PR22723 to compile identically (x86):

bool test(bool x, bool y) { return !x | !y; }
bool test(bool x, bool y) { return !x || !y; }
...
andb	%sil, %dil
xorb	$1, %dil
movb	%dil, %al
retq

Differential Revision: http://reviews.llvm.org/D12705

llvm-svn: 248634