SplitNode: Fix unecessary swizzle

[sunag]

Description

The idea is to force the optimization in case the user sets this unnecessarily.

After:
.. nodeVary3.xyz.xyz ..

// FLOW -> TransformedNormalView
nodeVar7 = cross( dpdy( nodeVary3.xyz.xyz ), normalize( nodeVary4 ) );
nodeVar8 = dpdx( nodeVary1.xy );
nodeVar9 = cross( normalize( nodeVary4 ), dpdx( nodeVary3.xyz.xyz ) );
nodeVar10 = dpdy( nodeVary1.xy );
...

Before:
.. nodeVary3.xyz ..

// FLOW -> TransformedNormalView
nodeVar7 = cross( dpdy( nodeVary3.xyz ), normalize( nodeVary4 ) );
nodeVar8 = dpdx( nodeVary1 );
nodeVar9 = cross( normalize( nodeVary4 ), dpdx( nodeVary3.xyz ) );
nodeVar10 = dpdy( nodeVary1 );
...

This contribution is funded by Google via Igalia

[sunag]

@LeviPesin I love yours revisions, thanks!