Prior to this CL, the regexp fast path check is stricter than it
needs to be. For example, adding any arbitrary property on the regexp
prototype would move the execution of all regexp builtins in the same
context onto the slow path. This actually happens in the real world:
popular web frameworks commonly monkey-patch builtin prototypes to add
functionality.

The intent of this CL is to widen the fast path for regexp builtins s.t.
modifications of the prototype that do not conflict with our
requirements stay on the fast path.

This is done by extending the current fast path check with an
additional step. If checking the prototype map identity or
relevant prototype property constness fails, we now compare the actual
value of all relevant properties against the expected value. If these
match, the prototype can be considered fast.

The new step as described in the previous paragraph is part of the
permissive fast path check (BranchIfFastRegExp_Permissive). The strict
variant (BranchIfFastRegExp_Strict) is also still required by a few
spots. We should refactor these to also allow the permissive check in
follow-up work.

Bug: v8:5577,chromium:977382
Change-Id: I69b2244e68ccfbd00edf17fc326aa4b5f5d089fa
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1706056
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62948}