Moving asserts from helpers to the call sites, so the failure messages are more
helpful.

BUG=v8:7308

Change-Id: I1da491d408c2e2a1017b1d3fe484b7b3d877a2cf
Reviewed-on: https://chromium-review.googlesource.com/1058802Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#53180}