The wasm compile fuzzer generated {br} instructions, but no {br_if} so
far. This CL adds that.

R=ahaas@chromium.org

Change-Id: Ib5e47a26d96e88498104e0d57b9a49b74b7356eb
Reviewed-on: https://chromium-review.googlesource.com/890450Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50950}