HLSL: add error for expected comparison sampler in SampleCmp* ops

This adds an error message if a non-comparison sampler is used with
comparison sampling methods.  There's no functional change for correct shaders.

Test/hlsl.samplecmp.negative2.frag

@@ -0,0 +1,10 @@
+
+Texture2D g_shadowTex;
+SamplerState g_shadowSampler;
+
+float4 main() : SV_Target0
+{
+    g_shadowTex.GatherCmpRed(g_shadowSampler, float2(0,0), 0, int2(0,0));  // ERROR (should be comparison sampler)
+
+    return 0;
+}