Fix cast from uniform struct to varying struct.

tests/struct-test-126.ispc

@@ -0,0 +1,23 @@
+
+export uniform int width() { return programCount; }
+
+
+struct Foo {
+    uniform float x;
+    varying float y;
+};
+
+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;
+    struct_1.y = aFOO[programIndex];
+    varying Foo struct_2 = struct_1;
+    RET[programIndex] = struct_2.x + struct_2.y;
+}
+
+
+export void result(uniform float RET[]) {
+    RET[programIndex] = 5 + programIndex + 1;
+}
+