Src access mask may be 0 with WAR barriers.

Bug: angleproject:5290
Change-Id: I798d4c36aa7d3d14db333a7e86794e13c8137f96
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2514841
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>