Only certain fiber types can have refs attached to them, so this moves the
Ref effect logic out of the common path and into the corresponding branch
of the layout phase's switch statement.
The types of fibers this affects are host components and class components.
Function components are not affected because they can only have a ref via
useImperativeHandle, which has a different implementation. The experimental
Scope type attaches its refs in the mutation phase, not the layout phase.
Andrew Clark
b8c96b136d