[wasm] [cleanup] Rename "checked" to "validate"

And reuse the CHECK_ERROR and VALIDATE macros.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: Ibeabdf0815418b6c70e2441ed9267261eb8883b6
Reviewed-on: https://chromium-review.googlesource.com/643131Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47711}