HLSL: Fix #954: Track/access subsets of flattened multi-level aggregates.

Works in conjuction with d1be754 to represent and modify a partially
dereferenced multi-level flattened aggregate.

Test/hlsl.flattenSubset2.frag

@@ -0,0 +1,24 @@
+struct Nested { float y; Texture2D texNested; };
+struct A { Nested n; float x; };
+struct B { Nested n; Texture2D tex; };
+
+Texture2D someTex;
+
+float4 main(float4 vpos : VPOS) : COLOR0
+{
+    A a1, a2;
+    B b;
+
+    // Assignment of nested structs to nested structs
+    a1.n = a2.n;
+    b .n = a1.n;
+
+    // Assignment of nested struct to standalone
+    Nested n = b.n; 
+
+    // Assignment to nestested struct members
+    a2.n.texNested = someTex;
+    a1.n.y = 1.0;
+
+    return float4(0,0,0,0);
+}