Remove one "mode" of LEB decoding by eliminating the {AdvancePCFlag},
and doing the PC advance in the caller instead.
The returned length is now always zero in case of an error, thus remove
the respective checks from the unit tests. The returned length does not
really matter if we ran into an error.

R=thibaudm@chromium.org

Change-Id: Ibfd94dd981cefa2fc24c7af560c85afd1c826f2c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2449972Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70404}