Fix regressions in asSeenFrom introduced in 3.7

The body of isLegalPrefix used to read:

    pre.isStable || !ctx.phase.isTyper

but was changed to drop the second condition in scala#21954 (originally included in
3.6.4-RC1, reverted from 3.6.4 final but back in 3.7). This has led to a
number of regressions, the last ones discussed in scala#23423.

To make the testcases added in scala#21594 pass, this PR proposes a less drastic
change: relax isLegalPrefix as before, unless we're doing an implicit search.
This should dramatically reduce the possibility for regressions.

Fixes scala#23423.
Fixes scala#22676.

[Cherry-picked 2e4bc0a][modified]

Author: tgodzik