With this CL, fast API calls reuse the same stack slot they are
using for the {fallback} parameter. This relies on the fact that
the fast calls are non-reentrant, due to their inability to call
into JavaScript.

Bug: chromium:1052746
Change-Id: I2c56fcbe425023244a566bb39439e8e04072f316
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2461729
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70409}