[InstSimplify] move (A & ~B) | (A ^ B) -> (A ^ B) from InstCombine

[InstSimplify] move (A & ~B) | (A ^ B) -> (A ^ B) from InstCombine

This is a straight cut and paste, but there's a bigger problem: if this
fold exists for simplifyOr, there should be a DeMorganized version for
simplifyAnd. But more than that, we have a patchwork of ad hoc logic
optimizations in InstCombine. There should be some structure to ensure
that we're not missing sibling folds across and/or/xor.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301213 91177308-0d34-0410-b5e6-96231b3b80d8