[InstSimplify] Handle (~A & ~B) | (~A ^ B) -> ~A ^ B

The code Sanjay Patel moved over from InstCombine doesn't work properly if the 'and' has both inputs as nots because we used a commuted op matcher on the 'and' first. But this will bind to the first 'not' on 'and' when there could be two 'not's. InstCombine could rely on DeMorgan to ensure the 'and' wouldn't have two 'not's eventually, but InstSimplify can't rely on that.

This patch matches the xor first then checks for the ands and allows a not of either operand of the xor.

Differential Revision: https://reviews.llvm.org/D32458

llvm-svn: 301329