Drop phase.isTyper use in isLegalPrefix/asf

Note that "asSeenFrom" (aka asf) is used by SymDenotation#findMember,
which is used by TypeComparer's "hasMatchingMember", as a part of
"compareRefinedSlow".

Previously (using the minimisation in i17222.8.scala) `summonOne` is
inlined during the "inlining" phase, while "summonInline" is inlined
during typing.  The result is that during inlining we fail to find
`Reader[BC, Int]` because we incorrectly consider `A{type F = Int}` as
stable.

[Cherry-picked 26ecda5][modified]

Author: tgodzik