Add a test for buffer auto-binding assignment.

Test/spv.buffer.autoassign.frag

@@ -0,0 +1,28 @@
+
+cbuffer MyUB1 : register(b5)  // explicitly assigned & offsetted
+{
+    float g_a;
+    int g_b;
+};
+
+cbuffer MyUB2  // implicitly assigned
+{
+    float g_c;
+};
+
+cbuffer MyUB3  // implicitly assigned
+{
+    float g_d;
+};
+
+struct PS_OUTPUT
+{
+    float4 Color : SV_Target0;
+};
+
+PS_OUTPUT main()
+{
+    PS_OUTPUT psout;
+    psout.Color = g_a + g_b + g_c + g_d;
+    return psout;
+}