This patch is the third one to support atomic functions on D3D11.

In this patch we enable support for atomic function returns outside of
assignments (e.g. part of arithmetic operations or to index into arrays) and
when used directly initialize a variable.

Note that we are still missing the functionality to be tag loops with
[allow_uav_condition] as required by InterlockedCompareExchange.

BUG=angleproject:2682
TEST=angle_end2end_tests

Change-Id: Ia409ebb10621fd5c514cf6c76f366a320a9d9fc1
Reviewed-on: https://chromium-review.googlesource.com/1208317Reviewed-by: Bryan Bernhart <bryan.bernhart@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>