Add clarifying comment.

Author: AlexVlx

llvm/lib/Target/SPIRV/SPIRVCallLowering.cpp

@@ -374,6 +374,11 @@ bool SPIRVCallLowering::lowerFormalArguments(MachineIRBuilder &MIRBuilder,
         buildOpDecorate(VRegs[i][0], MIRBuilder,
                         SPIRV::Decoration::FuncParamAttr, {Attr});
       }
+      // TODO: the AMDGPU BE only supports ByRef argument passing, thus for
+      //       AMDGCN flavoured SPIRV we CodeGen for ByRef, but lower it to
+      //       ByVal, handling the impedance mismatch during reverse
+      //       translation from SPIRV to LLVM IR; the vendor check should be
+      //       removed once / if SPIRV adds ByRef support.
       if (Arg.hasAttribute(Attribute::ByVal) ||
           (Arg.hasAttribute(Attribute::ByRef) &&
            F.getParent()->getTargetTriple().getVendor() ==