There's no restriction on the number of enable_if's in a
template-initializer, so simplify the compareThreeWay() constraints by
having separate constraints each for LeftType and RightType. Fewer
templates to instantiate = faster compile speed.
As a drive-by, drop remove_reference_t. We control all users and all of
them pass already-decayed types.
Change-Id: I17c01c7aa1ac03bb6db4b0bef1371ebc0641641d
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
091793ccaa