D3D does not define LOD in compute shader, so Sample()
is not supported, try to use SampleLevel().

Bug: angleproject:2756
TEST=angle_end2end_tests.ComputeShaderTest
     .TextureFunction/ES3_1_D3D11
     dEQP.GLES31/functional_image_load_store_2d_store*

Change-Id: I63e707d2b56e807cfe766cc21bc6b0819982ce80
Reviewed-on: https://chromium-review.googlesource.com/c/1379672
Commit-Queue: Xinghua Cao <xinghua.cao@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>