[wasm] [cleanup] Remove VALIDATE macro

It has exactly the inverse meaning of CHECK_ERROR. Since CHECK_ERROR is
used in the majority of the cases, and is more readable in most cases,
this CL replaces all uses of VALIDATE by CHECK_ERROR.
It also includes a few minor refactorings of the touched code.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I3d6b3b8bfd131e14f0d852b07622382b11b488f0
Reviewed-on: https://chromium-review.googlesource.com/643268Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47717}