The method does not do much, and all callers actually overwrite or
delete the stack slot right after calling this method anyways, so there
is no need to make the slot a stack slot first.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I4fd54d2ed5f86a3e011ddc2748833dc81052ef5b
Reviewed-on: https://chromium-review.googlesource.com/1111848Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53975}