Prune `PrefilterTree` edges instead of nodes.

Credit and gratitude to Daniel Keller for devising and refining the
probability-based heuristics.

Change-Id: Id99ffb0be27a2549f1ee0af1f7925027a3076127
Reviewed-on: https://code-review.googlesource.com/c/re2/+/60050
Reviewed-by: Perry Lorier <perryl@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed