HLSL: Add fall-back for opaque initializers to just generate long-term expected code.

This generated code needs an optimization pass to eliminate the assignments
to the opaque members.

Test/hlsl.flattenOpaqueInit.vert

@@ -6,8 +6,17 @@ float4 lookUp(FxaaTex tex)
     return tex.tex.Sample(tex.smpl, float2(0.3, 0.4));
 }
 
+FxaaTex fillOpaque()
+{
+    FxaaTex t;
+    t.smpl = g_tInputTexture_sampler;
+    t.tex = g_tInputTexture;
+    return t;
+}
+
 float4 main() : SV_TARGET0
 {
-    FxaaTex tex = { g_tInputTexture_sampler, g_tInputTexture };
-    return lookUp(tex);
+    FxaaTex tex1 = { g_tInputTexture_sampler, g_tInputTexture };
+    FxaaTex tex2 = fillOpaque();
+    return lookUp(tex1);
 }