[X86] Return Op instead of SDValue() for lowering flags_read/write intrinsics

Returning SDValue() means we didn't handle it and the common
code should try to expand it. But its a target intrinsic so
expanding won't do anything and just leave the node alone. But
it will print confusing debug messages.

By returning Op we tell the common code that the node is legal
and shouldn't receive any further processing.