HLSL: fix dereferencing when copying split structures with arrays

When copying split types with mixtures of user variables and buitins,
where the builtins are extracted, there is a parallel structures traversal.
The traversal was not obtaining the derefenced types in the array case.

gtests/Hlsl.FromFile.cpp

@@ -211,6 +211,7 @@ INSTANTIATE_TEST_CASE_P(
         {"hlsl.string.frag", "main"},
         {"hlsl.struct.split-1.vert", "main"},
         {"hlsl.struct.split.array.geom", "main"},
+        {"hlsl.struct.split.assign.frag", "main"},
         {"hlsl.struct.split.call.vert", "main"},
         {"hlsl.struct.split.nested.geom", "main"},
         {"hlsl.struct.split.trivial.geom", "main"},