[stubs] Cleanup binary op stubs.

... and introduce BinaryOpAssembler.

BUG=v8:6116

Change-Id: I86b0afedbe6ac11fda286b877fe55cda746f5347
Reviewed-on: https://chromium-review.googlesource.com/458278
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44029}