This CL adds support for memory operations to the AST-based Wasm compile
fuzzer. We've had several bugs in this area, so additional fuzz coverage should
help detect these sooner.

Change-Id: I28b5b95f1fc28939db764efe78de6d56bc61263c
Reviewed-on: https://chromium-review.googlesource.com/742383
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49144}