The barrier dependencies were set in two loops:

- From Previous to (Previous, Current]
- From (Previous, Current) to Current

The "Previous to Current" dependency was set in the first loop.  This
loop is conditioned to a previous barrier existing at all.

The bug is that for the first barrier node, the first loop does not
execute and the second loop does not create the dependency from the
first node (Previous index being 0 when no previous barrier exists) to
the barrier node.

Bug: angleproject:2361
Change-Id: I61a78dbbeba6cf97788bf8ea32908c7865ad48c2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1520994
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>