SI-8586 Pattern matcher analsis foiled by over-widening

In the enclosed test, the prefix checkable type
`ModuleTypeRef(F2.this, C)` was being inadvertently widened to
`ModuleTypeRef(F2[?], C)`. This started after some misguided
future-proofing in SI-6771 / 3009916.

This commit changes the `dealiasWiden` to a `delias`.