HLSL: Broaden solution for #940, editing integer input for 'flat'.

Test/hlsl.synthesizeInput.frag

@@ -0,0 +1,9 @@
+struct PSInput {
+  float interp;
+  uint no_interp;
+};
+
+float4 main(PSInput input : INPUT) : SV_TARGET
+{
+  return float4(float(input.no_interp), input.interp, 0, 1);
+}