HDRenderPipeline: Remove shadow warning about "loop 1 iteration"

ScriptableRenderPipeline/Core/CoreRP/ShaderLibrary/Shadow/Shadow.hlsl

     #define SHADOW_OPTIMIZE_REGISTER_USAGE   0
 #endif
 
+#if SHADOW_OPTIMIZE_REGISTER_USAGE == 1
+#   pragma warning(disable : 3557) // loop only executes for 1 iteration(s)
+#endif
+
 #include "CoreRP/Shadow/ShadowBase.cs.hlsl"	// ShadowData definition, auto generated (don't modify)
 #include "ShadowTexFetch.hlsl"						// Resource sampling definitions (don't modify)