Fix #1627: Allow constants as targets of dynamic calls.

ICSharpCode.Decompiler.Tests/TestCases/Pretty/DynamicTests.cs

@@ -410,6 +410,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			}
 		}
 
+		private static bool ConstantTarget(dynamic a)
+		{
+			return true.Equals(a);
+		}
+
 		private static dynamic GetDynamic(int i)
 		{
 			return null;

ICSharpCode.Decompiler/CSharp/ExpressionBuilder.cs

@@ -2891,7 +2891,6 @@ namespace ICSharpCode.Decompiler.CSharp
 		{
 			Debug.Assert(!argumentInfo.HasFlag(CSharpArgumentInfoFlags.NamedArgument));
 			Debug.Assert(!argumentInfo.HasFlag(CSharpArgumentInfoFlags.IsOut));
-			Debug.Assert(!argumentInfo.HasFlag(CSharpArgumentInfoFlags.Constant));
 
 			if (argumentInfo.HasFlag(CSharpArgumentInfoFlags.IsStaticType) && IL.Transforms.TransformExpressionTrees.MatchGetTypeFromHandle(inst, out var callTargetType)) {
 				return new TypeReferenceExpression(ConvertType(callTargetType))