Fix #980: flatten opaque initializers to use aliases.

Test/hlsl.flattenOpaqueInit.vert

@@ -0,0 +1,13 @@
+struct FxaaTex { SamplerState smpl; Texture2D tex; };
+SamplerState g_tInputTexture_sampler; Texture2D g_tInputTexture;
+
+float4 lookUp(FxaaTex tex)
+{
+    return tex.tex.Sample(tex.smpl, float2(0.3, 0.4));
+}
+
+float4 main() : SV_TARGET0
+{
+    FxaaTex tex = { g_tInputTexture_sampler, g_tInputTexture };
+    return lookUp(tex);
+}