HLSL: Allow Gather on integer textures in SM < 6.7

Texture2D<uint>.GatherRed() and similar Gather operations on integer
textures are valid in HLSL SM 5.0, but the non-float texture sampling
check rejected all non-float texture operations unconditionally.
Narrow the check to exclude Gather operations.

Author: xen2

spirv_hlsl.cpp

@@ -3952,7 +3952,7 @@ void CompilerHLSL::emit_texture_op(const Instruction &i, bool sparse)
 	else
 	{
 		auto &imgformat = get<SPIRType>(imgtype.image.type);
-		if (hlsl_options.shader_model < 67 && imgformat.basetype != SPIRType::Float)
+		if (hlsl_options.shader_model < 67 && imgformat.basetype != SPIRType::Float && !gather)
 		{
 			SPIRV_CROSS_THROW("Sampling non-float textures is not supported in HLSL SM < 6.7.");
 		}