Fix OpImageRead result type when compiling HLSL (#2706)

Fix OpImageRead result type when compiling HLSL

- Per the Vulkan spec, OpImageRead must return a 4-component vector always. When compiling HLSL, loads from a RWTexture of a template type with < 4 components would incorrectly generate an OpImageRead with a < 4 component result, resulting in validation errors.
- This was previously fixed for OpImageFetch in commit 4425f24; this commit does the same thing for OpImageRead.
- Added associated tests and expanded existing image fetch tests to check all the different types of textures, in both float and int incarnations, for completeness.
- Update other HLSL tests involving OpImageRead

gtests/Hlsl.FromFile.cpp

@@ -235,6 +235,7 @@ INSTANTIATE_TEST_SUITE_P(
         {"hlsl.groupid.comp", "main"},
         {"hlsl.identifier.sample.frag", "main"},
         {"hlsl.if.frag", "PixelShaderFunction"},
+        {"hlsl.imageload-subvec4.comp", "main"},
         {"hlsl.imagefetch-subvec4.comp", "main"},
         {"hlsl.implicitBool.frag", "main"},
         {"hlsl.inf.vert", "main"},