InstCombine: ((A | ~B) ^ (~A | B)) to A ^ B

Proof using CVC3 follows:
$ cat t.cvc
A, B : BITVECTOR(32);
QUERY BVXOR((A | ~B),(~A |B)) = BVXOR(A,B);
$ cvc3 t.cvc
Valid.

Patch by Mayur Pandey!

Differential Revision: http://reviews.llvm.org/D4883

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215621 91177308-0d34-0410-b5e6-96231b3b80d8