fix #30394, an unsoundness in ml_matches (#30396)
This fixes a corner case where a bug is caused, counter-intuitively,
by an over-estimated intersection.
We have method signatures A and B, with A<B (A is a strict subtype).
We have a dispatch tuple X, where X<:B and !(X<:A).
However, intersection returns X for intersect(X,A). Since there
appears to be a match there and A<B, ml_matches skips the match with B.
The fix just requires dispatch tuples to be a subtype of a signature
in order to match at all.
