Seperate vertex and fragment shader read/write with shader stage
specific barriers

Bug: angleproject:4467
Change-Id: Id18909b497b087e58d750023399943bc9d414edf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2102957
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>