NME: Fix crash when iridescence and clearcoat are used at the same time

[Popov72]

See https://forum.babylonjs.com/t/node-material-compilation-error-with-iridescence-block/57401

[bjsplat]

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To prevent this PR from going to the changelog marked it with the "skip changelog" label.

[bjsplat]

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[bjsplat]

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[bjsplat]

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