All microbenchmarks now add 20 variables together per iteration, rather than
just a single variable.

Also re-add a sanity check after the loop, and fix a missing variable add (a15)
from the loop.

Bug: v8:1569
Change-Id: Ie54357b5cedaafd85f01c699c08b24a5ee6468c9
Reviewed-on: https://chromium-review.googlesource.com/636284Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47618}