[wasm] Clean up and speed up compile fuzzer

Instead of calling a number of member functions which return lambdas
which are then wrapped in std::functions, just use the member functions
directly. This allows to make the arrays with the alternatives constexpr
instead of dynamically filling it on each call.

R=eholk@chromium.org, ahaas@chromium.org

Change-Id: Id1256f442f411eb291941911b25de24a985a9b34
Reviewed-on: https://chromium-review.googlesource.com/753722
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Eric Holk <eholk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49146}