HLSL: Convert run-time sampler assignments to compile-time aliases.

For "s.m = t", a sampler member assigned a sampler, make t an alias
for s.m, and when s.m is flattened, it will flatten to the alias t.
Normally, assignments to samplers are disallowed.

Test/hlsl.aliasOpaque.frag

@@ -0,0 +1,29 @@
+struct OS {
+    SamplerState ss;
+    float a;
+    Texture2D tex;
+};
+
+SamplerState gss;
+SamplerState gss2;
+Texture2D gtex;
+
+float4 osCall(OS s)
+{
+    return s.a * s.tex.Sample(s.ss, float2(0.2, 0.3));
+}
+
+float4 main() : SV_TARGET0
+{
+    OS os;
+    os.ss = gss2;
+    os.ss = gss;
+    os.tex = gtex;
+    os.a = 3.0;
+
+    // this should give an error
+    //SamplerState localss;
+    //localss = gss2;
+
+    return osCall(os);
+}