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.flattenSubset.frag

@@ -0,0 +1,32 @@
+struct S0
+{
+    int x;
+    int y;
+    SamplerState ss;
+};
+
+struct S1
+{
+    float b;
+    SamplerState samplerState;
+    S0 s0;
+    int a;
+};
+
+struct S2
+{
+    int a1;
+    int a2;
+    int a3;
+    int a4;
+    int a5;
+    S1 resources;
+};
+
+float4 main(float4 vpos : VPOS) : COLOR0
+{
+    S1 s1;
+    S2 s2;
+    s2.resources = s1;
+    return float4(0,0,0,0);
+}