Prune `PrefilterTree` edges instead of nodes.
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 <d2756efe887c28699f4587ef278926ee3b3bfae5@google.com>
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2