Image load and store are incoherent memory access
operations, so need to use memoryBarrier to ensure
the visibility of incoherent memory access operations
with other operations.

Bug: angleproject:3044
Change-Id: I4d474d260a3199c026911de8d20878cd327d16c6
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1677420
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>