[X86] Replace explicit constant handling in sub(C1, xor(X, C2)) -> add(xor(X,...

[X86] Replace explicit constant handling in sub(C1, xor(X, C2)) -> add(xor(X, ~C2), C1+1) fold. NFCI.

NFC cleanup before adding vector support - rely on the SelectionDAG to handle everything for us.