Does a couple of cleanups on interpreter assembler:
 - Adding naming to the variable fields to improve debugability
 - Grouping functions which deal with loading the state passed between
   bytecode handlers (e.g. bytecode array / offset / etc.).
 - Fix some comments in interpreter-generator.cc

Change-Id: I9decefebbdf7830a7ce75dd46e8a69a1db3c4cc8
Reviewed-on: https://chromium-review.googlesource.com/625797Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47499}