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.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215524 91177308-0d34-0410-b5e6-96231b3b80d8