fix subset isa method on a subset.

[dwarring]

perl6 -e'subset S of Int; subset S2 of S; say S2.isa(S)'

now returns True. fully resolves RT #132073

[zoffixznet]

Does perl6 -e'subset S of Int; subset S3 of S; subset S2 of S; say S2.isa(S3)' return False?

Not very familiar with this part of codebase, but looks like the added condition simply checks whether the given type is a subset… any subset.

[dwarring]

Yes, that returns False.
I borrowed the added logic from the type_check method

[zoffixznet]

👍 Thanks!