[regexp] Increase number of backtracks before fallback

This commit increases the number of backtracks before we fall back to
the experimental engine.  The previous limit was reached just before
irregexp finished in sunspider/string-tagcloud and
web-tooling-benchmark/coffeescript.  This meant that we aborted
execution and started over with a slower (but asymptotically faster)
engine just before we would've finished, causing a performance
regression.

Bug: chromium:1138975,v8:10765
Change-Id: I10a4b149300f88680084604310730ee91758bfa0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2479010
Commit-Queue: Martin Bidlingmaier <mbid@google.com>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70673}

src/flags/flag-definitions.h

@@ -1503,7 +1503,7 @@ DEFINE_BOOL(trace_experimental_regexp_engine, false,
 DEFINE_BOOL(enable_experimental_regexp_engine_on_excessive_backtracks, true,
             "fall back to a breadth-first regexp engine on excessive "
             "backtracking")
-DEFINE_UINT(regexp_backtracks_before_fallback, 10000,
+DEFINE_UINT(regexp_backtracks_before_fallback, 50000,
             "number of backtracks during regexp execution before fall back "
             "to experimental engine if "
             "enable_experimental_regexp_engine_on_excessive_backtracks is set")