Ensure that GetUnusedRegister is always called before
acquiring the scratch register in case it is needed
for spilling the value of the used register.

Bug: v8:6600, chromium:910824
Change-Id: I93ae684ad504584807dfa6227b6af14609c6bcf5
Reviewed-on: https://chromium-review.googlesource.com/c/1387498Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58442}