Fix for struct with undound members

tests/struct-test-128.ispc

@@ -0,0 +1,21 @@
+
+export uniform int width() { return programCount; }
+
+
+struct Foo {
+    float x;
+};
+
+export void f_fu(uniform float RET[], uniform float aFOO[], uniform float b) {
+    float a = aFOO[programIndex];
+    uniform Foo struct_1;
+    struct_1.x = b;
+    varying Foo struct_2 = struct_1;
+    RET[programIndex] = struct_2.x;
+}
+
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 5;
+}
+