Remainder of inout signature element not passed through when one component is modified

In the case of an inout signature element, we do not pass through components not modified by the shader unless no components are touched.

The following test case succeeds if you do not write to pos.w, but fails with a validation error if you do since we do not output pos.xyz. Fxc does pass these through, so we should fix this.

// RUN: %dxc -E main -T vs_6_0 %s | FileCheck %s

// CHECK: call void @dx.op.storeOutput.f32(i32 5, i32 0, i32 0, i8 0,
// CHECK: call void @dx.op.storeOutput.f32(i32 5, i32 0, i32 0, i8 1,
// CHECK: call void @dx.op.storeOutput.f32(i32 5, i32 0, i32 0, i8 2,
// CHECK: call void @dx.op.storeOutput.f32(i32 5, i32 0, i32 0, i8 3,

// Should pass the rest (xyz) of SV_Position through
void main(inout float4 pos: SV_Position) {
  pos.w = 1;
}

Start by reproducing the issue with the provided HLSL test using %dxc -E main -T vs_6_0 and inspect the FileCheck output and validation error. The fix is complete when modifying only pos.w still produces storeOutput calls for all four SV_Position components, including pos.xyz, as expected by the checks.