These counters track how often the slow path of these two builtins is
hit. Exec is very permissive, its fast-path check doesn't look at the
regexp prototype at all. Replace is strict; any change on the
prototype will trigger the slow path.

Chromium CL: https://crrev.com/c/1875250

Bug: v8:5577
Change-Id: I9807f43829981445b75b5c5d29800cbdac9bc26a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1873698Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64496}