Because D3D11 makes a coherent write, so memoryBarrier is not needed on
D3D backend.
This patch also simples some test cases and modifies some cases errors when
accessing memory after an incoherent write, but does not use memoryBarrier.

BUG=angleproject:2280
TEST=angle_end2end_tests.ComputeShaderTest.*

Change-Id: Iee5d105a0b6d8534aded03fdaaefc909864d33a4
Reviewed-on: https://chromium-review.googlesource.com/937023Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>