Fix #19789: Merge same TypeParamRef in orDominator

[noti0na1]

Fix #19789

Merge refined or applied types if they share the same TypeParamRef as tycon in orDominator.

[odersky]

I think this can be optimized by adding a first clause

 if tp1 eq tp2 then tp1

Also, instead of comparing TypeParamRefs directly we could just do it like this:

case tp1: TypeParamRef if tp1 == tp2 => tp1

Note the first test might still need the second, since hash-consing is not 100% assured (e.g. we might get copies after a table overflow). So exploiting hash consing should always be an optimization only.