[SPIR-V] Not all unnecessary bindings are eliminated using SPIR-V backend

Consider following HLSL

struct PixelInput
{
	float4 pos : SV_Position0;
	float2 texCoord : TEXCOORD0;
};

Texture2D Tex0;
SamplerState SS0;
Texture2D Tex1;
SamplerState SS1;

float4 main(in PixelInput In) : SV_Target0
{
	const float2 val = Tex0.Sample(SS0, In.texCoord).xy;
	if (((val.x == 0.0f) && (val.y == 0.0f)))
	{
		clip(-1.0f);
	}
	clip(-0.5f);
	return Tex1.Sample(SS1, In.texCoord);
}

and following compilation command: dxc -T ps_6_0 -spirv test.hlsl -Fo test.spv
Output of the spirv-dis test.spv will be

; SPIR-V
; Version: 1.0
; Generator: Google spiregg; 0
; Bound: 35
; Schema: 0
               OpCapability Shader
               OpMemoryModel Logical GLSL450
               OpEntryPoint Fragment %main "main" %gl_FragCoord %in_var_TEXCOORD0 %out_var_SV_Target0
               OpExecutionMode %main OriginUpperLeft
               OpSource HLSL 600
               OpName %type_2d_image "type.2d.image"
               OpName %Tex0 "Tex0"
               OpName %type_sampler "type.sampler"
               OpName %SS0 "SS0"
               OpName %in_var_TEXCOORD0 "in.var.TEXCOORD0"
               OpName %out_var_SV_Target0 "out.var.SV_Target0"
               OpName %main "main"
               OpName %type_sampled_image "type.sampled.image"
               OpDecorate %gl_FragCoord BuiltIn FragCoord
               OpDecorate %in_var_TEXCOORD0 Location 0
               OpDecorate %out_var_SV_Target0 Location 0
               OpDecorate %Tex0 DescriptorSet 0
               OpDecorate %Tex0 Binding 0
               OpDecorate %SS0 DescriptorSet 0
               OpDecorate %SS0 Binding 1
      %float = OpTypeFloat 32
    %float_0 = OpConstant %float 0
%type_2d_image = OpTypeImage %float 2D 2 0 0 1 Unknown
%_ptr_UniformConstant_type_2d_image = OpTypePointer UniformConstant %type_2d_image
%type_sampler = OpTypeSampler
%_ptr_UniformConstant_type_sampler = OpTypePointer UniformConstant %type_sampler
    %v4float = OpTypeVector %float 4
%_ptr_Input_v4float = OpTypePointer Input %v4float
    %v2float = OpTypeVector %float 2
%_ptr_Input_v2float = OpTypePointer Input %v2float
%_ptr_Output_v4float = OpTypePointer Output %v4float
       %void = OpTypeVoid
         %20 = OpTypeFunction %void
%type_sampled_image = OpTypeSampledImage %type_2d_image
       %bool = OpTypeBool
       %Tex0 = OpVariable %_ptr_UniformConstant_type_2d_image UniformConstant
        %SS0 = OpVariable %_ptr_UniformConstant_type_sampler UniformConstant
%gl_FragCoord = OpVariable %_ptr_Input_v4float Input
%in_var_TEXCOORD0 = OpVariable %_ptr_Input_v2float Input
%out_var_SV_Target0 = OpVariable %_ptr_Output_v4float Output
       %main = OpFunction %void None %20
         %22 = OpLabel
         %23 = OpLoad %v2float %in_var_TEXCOORD0
         %24 = OpLoad %type_2d_image %Tex0
         %25 = OpLoad %type_sampler %SS0
         %26 = OpSampledImage %type_sampled_image %24 %25
         %27 = OpImageSampleImplicitLod %v4float %26 %23 None
         %28 = OpCompositeExtract %float %27 0
         %29 = OpFOrdEqual %bool %28 %float_0
         %30 = OpCompositeExtract %float %27 1
         %31 = OpFOrdEqual %bool %30 %float_0
         %32 = OpLogicalAnd %bool %29 %31
               OpSelectionMerge %33 None
               OpBranchConditional %32 %34 %33
         %34 = OpLabel
               OpKill
         %33 = OpLabel
               OpKill
               OpFunctionEnd

Tex1 and SS1 were eliminated (as unreachable I guess) but Tex0 and SS0 are still here while results of the sampling are not really used -- both branches end up in OpKill, so sampling could be removed and so Tex0 and SS0 could be removed too... ...and whole shader could be collapsed to the single OpKill 😄

Tested on dxc_2021_07_01.

Start by reproducing the issue with dxc -T ps_6_0 -spirv test.hlsl -Fo test.spv, then inspect the SPIR-V backend's handling of dead code and resource elimination. Use spirv-dis on the output to verify that the unused sampling and bindings are removed and that the shader is reduced to the terminating operation.