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.

Differential Revision: http://reviews.llvm.org/D4898

llvm-svn: 215974