Revert of [regexp] Remove IsRegExp intrinsic (patchset #1 id:1 of https://codereview.chromium.org/2591923003/ )

Reason for revert:
speculative revert: https://codereview.chromium.org/2596013002/

Original issue's description:
> [regexp] Remove IsRegExp intrinsic
>
> The two remaining uses of this intrinsic in debug.js and mirrors.js now
> simply rely on the runtime function.
>
> BUG=v8:5339
>
> Review-Url: https://codereview.chromium.org/2591923003
> Cr-Commit-Position: refs/heads/master@{#41892}
> Committed: https://chromium.googlesource.com/v8/v8/+/c9cb94a06fa7a863d24dd6760b66cecd55748abf

TBR=bmeurer@chromium.org,jgruber@chromium.org
# Skipping CQ checks because original CL landed less than 1 days ago.
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
BUG=v8:5339

Review-Url: https://codereview.chromium.org/2592383002
Cr-Commit-Position: refs/heads/master@{#41915}