In WebAssembly benchmarks I saw Binop instructions with 6 inputs. We
don't know how many inputs there can actually be, so we conservatively
increase the number to 8 now.

R=jarin@chromium.org
Bug=chromium:842501

Change-Id: Id087481e7e524006c2f03fc545f9e35d1cad1fe8
Reviewed-on: https://chromium-review.googlesource.com/1061114Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53207}