skipping narrowing if the old algorithm produces a type to which the
assigned type is not assignable.
This also means we'll no longer narrow for erroneous assignments where
the assigned type is not assignable to the declared type. This is the
reason for the numericLiteralTypes3 baseline change.
Fixes#26405.
typeMaybeAssignableTo.
typeMaybeAssignableTo decomposed unions at the top level of the assigned
type but didn't properly handle other unions that arose during
assignability checking, e.g., in the constraint of a generic lookup
type.
Fixes#26130.
