Fix #2987: `(nuint)(-3)` is not a compile-time constant.

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

@@ -206,6 +206,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			return (nint)(0 - x);
 		}
 
+		public bool CompareToMinus3(nuint x)
+		{
+			return x == unchecked((nuint)(-3));
+		}
+
 		public nint SignedNotFittingIn32Bits()
 		{
 			// Explicit `unchecked` is necessary when casting oversized constant to nint

ICSharpCode.Decompiler/CSharp/Resolver/CSharpResolver.cs

@@ -1223,14 +1223,11 @@ namespace ICSharpCode.Decompiler.CSharp.Resolver
 				ResolveResult rr = ResolveCast(targetType, expression);
 				if (rr.IsError)
 					return rr;
-				Debug.Assert(rr.IsCompileTimeConstant);
-				return new ConstantResolveResult(nullableType, rr.ConstantValue);
-			}
-			else
-			{
-				return Convert(expression, nullableType,
-							   isNullable ? Conversion.ImplicitNullableConversion : Conversion.ImplicitNumericConversion);
+				if (rr.IsCompileTimeConstant)
+					return new ConstantResolveResult(nullableType, rr.ConstantValue);
 			}
+			return Convert(expression, nullableType,
+							isNullable ? Conversion.ImplicitNullableConversion : Conversion.ImplicitNumericConversion);
 		}
 		#endregion