Fixes several warnings reported for internal repo by:
  * using vector::empty instead of vector::size() == 0
  * removing redundant return; at the end of a function
  * making operator= return OriginalType&

Bug: v8:9183
Change-Id: I8c725bd7b0bc011557fb2bb68a561ee413ab38f5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1589978
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61149}