kScratchRegister is not guaranteed to be preserved across calls to
RecordWriteField.

Bug: v8:6844
Change-Id: I65377852807f52be821be4d5911b07886102c5ee
Reviewed-on: https://chromium-review.googlesource.com/709114Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48427}