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.

Test/hlsl.struct.split.assign.frag

@@ -0,0 +1,12 @@
+struct S {
+    float f;
+    float4 pos : SV_Position;
+};
+
+float4 main(int i, S input[3]) : COLOR0
+{
+    S a[3];
+    input = a;
+
+    return float3(1.0);
+}