Opportunistically merge A & A -> A and B | B -> B in <:<
Opportunistically merge A & A -> A and B | B -> B in <:<

Cases like these (in fact one of the operads was a type variable
the other its underlying polyparam) arose in pos/overloads.scala and caused
deep subtype recursions.

lampepfl/dotty,sjrd/dotty,som-snytt/dotty,dotty-staging/dotty,reactormonk/dotty,dotty-staging/dotty,lampepfl/dotty,sjrd/dotty,sjrd/dotty,dotty-staging/dotty,dotty-staging/dotty,lampepfl/dotty,som-snytt/dotty,som-snytt/dotty,sjrd/dotty,som-snytt/dotty,som-snytt/dotty,dotty-staging/dotty,densh/dotty,sjrd/dotty,VladimirNik/dotty,lampepfl/dotty,densh/dotty,lampepfl/dotty,reactormonk/dotty,VladimirNik/dotty