Fix a DAG combine bug visitBRCOND() is transforming br(xor(x, y)) to br(x != y).
It cahced XOR's operands before calling visitXOR() but failed to update the
operands when visitXOR changed the XOR node.

rdar://12968664


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171999 91177308-0d34-0410-b5e6-96231b3b80d8