HLSL: Handle flatten for reads from flatten structs and parameter passing.

Test/hlsl.entry-in.frag

@@ -0,0 +1,13 @@
+struct InParam {
+    float2 v;
+    float4 fragCoord : SV_POSITION;
+    int2 i2;
+};
+
+float4 PixelShaderFunction(InParam i) : COLOR0
+{
+    InParam local;
+    local = i;
+
+    return local.fragCoord;
+}