Also refactor the implementation of i32.eqz such that the same
platform-specific code can be reused.
As a next step, it should be straight-forward to add other i32
comparison operations.

R=titzer@chromium.org

Bug: v8:6600
Change-Id: I4e8768d4ceb7294ba35777b7777ddd69d1a58cf1
Reviewed-on: https://chromium-review.googlesource.com/877889
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50868}