There were a couple of low-hanging fruits in code-stub-assembler. Tried
to keep it short to avoid conflicts with other CLs.

Bug: v8:9810
Change-Id: If23e16019116c22ddd6282867d9dd0b2e65a23f0
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1906570
Commit-Queue: Santiago Aboy Solanes <solanes@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64890}