The return type of stores is void according to the WebAssembly spec.
In wasm-opcodes.h, however, the return type of stores was set to the
value type. This CL changes the signature of stores, and fixes some
issues which were caused because now there exist opcodes with a return
count different to 1.

R=titzer@chromium.org, clemensh@chromium.org

Bug: v8:7109
Change-Id: I94d1b049e6d0032868b2ce83f52e16b474de3dea
Reviewed-on: https://chromium-review.googlesource.com/803495Reviewed-by: Ben Titzer <titzer@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49836}