This reduces reduces code size by 16 KB while keeping the fast path (in
which the given argument is already a smi) inlined and extracting
remaining logic to a stub call.

Change-Id: I531999c990519eef1247cc3785ad4b16164f7a5e
Reviewed-on: https://chromium-review.googlesource.com/833912Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50224}