Regression: Top level inputs not working in code generation

[kwokcb]

Issue

A top-level input is not handled properly for code generation anymore.
This is came up before and was fixed (think around 1.38.9/10 time-frame) but appears to be broken again in main.

Steps

Load the resources/Materials/TestSuite/stdlib/nodegraphs/top_level_input.mtlx test file.

<?xml version="1.0"?>
<materialx version="1.39" colorspace="lin_rec709">
  <surfacematerial name="surfacematerial" type="material" xpos="17.246376" ypos="-15.689655">
    <input name="surfaceshader" type="surfaceshader" nodename="surface_unlit" />
  </surfacematerial>
  <input name="input_color3" type="color3" value="0, 0, 1" xpos="10.905797" ypos="-15.982759" />
  <surface_unlit name="surface_unlit" type="surfaceshader" xpos="14.231884" ypos="-16.241379">
    <input name="emission_color" type="color3" interfacename="input_color3" />
  </surface_unlit>
</materialx>

It may load properly initially in the desktop node editor (render blue which is the top level input colour), but then if you re-evaluate it, or modify any value it will become while again.

Looking at the output code, the top-level input does not seem to be considered anymore.