improve check syntax so opening the context-sensitive menu is faster
in certain situations

Specifically, don't compute the identifiers-hash before opening the
menu but, when it does need to be computed, avoid consider the same
arrows over and over. This makes an especially big difference when a
region of text is selected that contains lots of arrow heads that all
point to a place that has lots of arrows.

Specifically, the time to compute the identifers hash for the example in
the PR is now about 200x faster (altho this still takes 200 msec, which
is why it isn't computed at all in that sequence of steps any more).

closes PR 14586