The Wasm AST-based fuzzer is supposed to create valid modules by
construction. This change adds a CHECK to enforce this property.

Additionally, this change exposed several cases where we were not generating
valid modules before:
  * Block types did not match up correctly
  * Memory operations could have invalid alignments
  * Storing an i64 could generate an i32 argument incorrectly.
This CL includes fixes for these issues as well.

Bug: 
Change-Id: I1aef5532bc880367ec46dc6e79b2d4dbacf2f84b
Reviewed-on: https://chromium-review.googlesource.com/757129
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49241}