HLSL: add test coverage for sub-vec4 texture intrinsics

This changes no functional code.  There was a bit of a testing hole
in that textures templatized on sub-vec4 types were not being exercised
with any intrinsics.  This adds some basic sanity coverage of that case.

gtests/Hlsl.FromFile.cpp

@@ -271,6 +271,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.structin.vert", "main"},
         {"hlsl.structIoFourWay.frag", "main"},
         {"hlsl.structStructName.frag", "main"},
+        {"hlsl.texture.subvec4.frag", "main"},
         {"hlsl.this.frag", "main"},
         {"hlsl.intrinsics.vert", "VertexShaderFunction"},
         {"hlsl.intrinsic.frexp.vert", "VertexShaderFunction"},