The accumulator should never be alive when jumping back to a loop
header, or jumping out of a loop. This means that as far as far as
TurboFan is concerned, we never need to create Phis or LoopExitValues
for the accumulator, as its value should not escape the loop.

For safety, this also augments the IsLivenessValid DCHECK in the
liveness analysis to check that the accumulator is not live in these
cases, and amends the bytecode analysis tests to kill the accumulator
where necessary to ensure this.

As a drive-by, added some comments to the more complex bytecode analysis
tests, since figuring out what they were for and how to fix them took a
non-trivial amount of time.

Change-Id: Idecf76a36681d724134c59768650c23cc6b0e9ef
Reviewed-on: https://chromium-review.googlesource.com/615168
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47388}