HLSL: iomapper: Fix #914. Tolerate user aliasing of bindings.

Because it is valid in HLSL to alias bindings:
A) remove validation that aliasing is not done
B) make the algorithms tolerate aliasing

Test/spv.ssboAlias.frag

@@ -0,0 +1,10 @@
+AppendStructuredBuffer<uint> Buf1 : register(u1);
+AppendStructuredBuffer<uint> Buf2 : register(u2);
+AppendStructuredBuffer<uint> Buf3 : register(u1);
+
+float4 main() : SV_Target
+{
+	Buf1.Append(10u);
+	Buf2.Append(20u);
+	return float4(1.0, 3.0, 5.0, 1.0);
+}