Recognize and avoid spending compile time on "megamorphic" clusters

Megamorphic clusters are clusters which never stabilize to have a fixed number
of shape signatures.  Once we're reasonably sure a cluster is megamorphic we
stop compiling it forever.

This is necessarily a fuzzy concept, and I suspect we'll have to adjust our
exact definition of megamorphic with time.

PiperOrigin-RevId: 220317226