Implements the saturating opcode i64.trunc_s:sat/f32.

Also does some refactoring of the i32 saturating opcodes use a simplier
solution (calling a single method to handle all i32 values).

Also refactors code so that the remaining i64 saturating conversions
should be easy to add to the wasm compiler.

Bug: v8:7226
Change-Id: I031aca1e059b4baa989a56ecbc16941f591ff9b3
Reviewed-on: https://chromium-review.googlesource.com/887333
Commit-Queue: Karl Schimpf <kschimpf@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51001}