InstCombine: Combine (xor (or %a, %b) (xor %a, %b)) to (add %a, %b)

Correctness proof of the transform using CVC3-

$ cat t.cvc
A, B : BITVECTOR(32);
QUERY BVXOR(A | B, BVXOR(A,B) ) = A & B;

$ cvc3 t.cvc
Valid.

llvm-svn: 215524