Add clarifying comments for default vtx/prim IDs (#6539)

MS outputs require vertex or primitive ID parameters and so do not
require an undefined value to be set to initialize the parameter value
This just adds comments to clarify the potentially confusing code.

Fixes #6486

Author: Greg Roth

lib/HLSL/HLSignatureLower.cpp

@@ -1138,6 +1138,7 @@ void HLSignatureLower::GenerateDxilInputsOutputs(DXIL::SignatureKind SK) {
     DXASSERT_NOMSG(0);
   }
   bool bInput = SK == DXIL::SignatureKind::Input;
+  // vertex or primitive IDs are required for GS/DS/HS inputs and MS outputs
   bool bNeedVertexOrPrimID =
       bInput && (props.IsGS() || props.IsDS() || props.IsHS());
   bNeedVertexOrPrimID |= !bInput && (props.IsMS() || props.IsMeshNode());
@@ -1154,6 +1155,9 @@ void HLSignatureLower::GenerateDxilInputsOutputs(DXIL::SignatureKind SK) {
 
   Constant *constZero = hlslOP->GetU32Const(0);
 
+  // Default value of vertexidx mainly used for LoadInput where it's optional
+  // Mesh outputs require this to be set and so need no default.
+  // They will be set correctly in GenerateInputOutputUserCall
   Value *undefVertexIdx = props.IsMS() || props.IsMeshNode() || !bInput
                               ? nullptr
                               : UndefValue::get(Type::getInt32Ty(HLM.GetCtx()));