This patch implements translating textureGather[Offset] into HLSL
when the sampler is a shadow sampler. The related HLSL function
should be GatherCmp().

According to the definition of textureGatherOffset():
([ESSL 3.1] Chapter 8.9.3 Page 138)
- gvec4 textureGatherOffset(gsampler2D sampler, vec2 P,
  ivec2 offset, [,int comp])
- vec4 textureGatherOffset(sampler2DShadow sampler, vec2 P,
  float refZ, ivec2 offset)
We need to add parameter "refZ" before "offset" when the sampler
is a shadow sampler.

Bug: angleproject:2826
Test: dEQP-GLES31.functional.texture.gather.basic.2d.depth32f.*
     dEQP-GLES31.functional.texture.gather.basic.2d_array.depth32f.*
     dEQP-GLES31.functional.texture.gather.basic.cube.depth32f.*
     dEQP-GLES31.functional.texture.gather.offset.min_required_offset.2d.depth32f.*
     dEQP-GLES31.functional.texture.gather.offset.min_required_offset.2d_array.depth32f.*
     dEQP-GLES31.functional.texture.gather.offset.implementation_offset.2d.depth32f.*
     dEQP-GLES31.functional.texture.gather.offset.implementation_offset.2d_array.depth32f.*

Change-Id: I9a7d095dd3cfa41aaefd14d012ed1f309abfc6d5
Reviewed-on: https://chromium-review.googlesource.com/c/1244081
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>