Optimize widen
Optimize widen

For typer/*.scala, we had 3.4M calls to widen, of which
almost 3M were to the three cases that are now handled specially.

dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,sjrd/dotty,lampepfl/dotty,lampepfl/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,lampepfl/dotty