The only users of the LoadStoreOpcodeOf function were a number of
macros in wasm-macro-gen.h, and three test functions using it directly.
This CL refactors those functions to also use the macros.
In one case, this requires storing the value in a local variable first.

R=ahaas@chromium.org

Change-Id: Ia2fbf67a3831fafc9345e155eb240cf1bf6feb5d
Reviewed-on: https://chromium-review.googlesource.com/486842Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44885}