This makes sure the {kScratchRegister} is not used across macro
instructions (e.g. {LeaveFrame}) that would clobber its content.
Generally it is highly unsafe to use such scratch registers with a
life-range spanning macro instructions.

R=neis@chromium.org

Review-Url: https://codereview.chromium.org/2521973002
Cr-Commit-Position: refs/heads/master@{#41181}