HLSL: Accept SV_Cull/ClipDistanceN, by refactoring the way semantics are mapped.

Test/hlsl.semantic.vert

@@ -0,0 +1,15 @@
+struct S {
+    float clip  : SV_ClipDistance;
+    float clip0 : SV_ClipDistance0;
+    float clip7 : SV_ClipDistance7;
+    float cull  : SV_CullDistance;
+    float cull2 : SV_CullDistance2;
+    float cull5 : SV_CullDistance5;
+    int ii      : SV_InstanceID;
+};
+
+S main(S ins)
+{
+    S s;
+    return s;
+}