Change heuristics around creating small clusters

 - Don't count "Const" nodes in cluster size
 - Bump up the default minimum cluster size to 4

These changes are motivated by some internal auto-clustering benchmarks.

Some tests had to be updated to work with these new heuristics.  I've tried to
ensure that the tests keep testing what they intended to test.

However, these changes do demonstrate a weakness in our tests; the tests not
specifically testing our clustering heuristics should be more resilient to
changes to our clustering heuristics.

PiperOrigin-RevId: 238344897