Rollback constraints if isSameType failed second direction

[EugeneFlesselle]

It turns out the following assertion does not hold in the current definition of isSameType

val preConstraint = constraint
val isSame = isSubType(tp1, tp2) && isSubType(tp2, tp1)
isSame.ensuring(_ || constraint == preConstraint)

I didn't try to form a minimised snippet where this would cause a problem. But as an example, the code in #19955 (comment) produces invalid constraints which lead to suspicious looking <notypes>s in the subtyping trace.