[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.
 

llvm-svn: 301213