Simple performance improvement for Denotations

[noti0na1]

A simple performance improvement extracted from #21278

This PR:

• caches signature in SingleDenotation for matchDegree;
• reduces denot calls in widens.

This is a simpler approach which can reduce the compilation time of #20217 from 60s to about 20s 40s.

I revert the signature cache to keep this PR simple, because I don't understand why it works.

[noti0na1]

Probably the question is why we don't cache signatures for provisional MethodOrPoly.

I add an assertion, but the signature never change even if the type is provisional.