Fix #3702: Don't rewrite GetValueOrDefault() to == true in ?. chains

Turning bool?.GetValueOrDefault() into `== true` ends an enclosing
null-conditional chain, so `c?.Flag.GetValueOrDefault()` became
`(c?.Flag == true)` of type bool instead of bool?. A following `?? false`
then fails to compile (CS0019), and a following `.ToString()` yields
"False" instead of null when c is null. Keep the call when its target is
part of a ?. chain.

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

@@ -32,6 +32,7 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			public readonly MyStruct ReadonlyStructField;
 			public string Text;
 			public MyClass Field;
+			public bool? NullableBool;
 			public MyClass Property { get; set; }
 			public MyClass this[int index] => null;
 			public MyClass Method(int arg)
@@ -380,5 +381,15 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 			return x?.ToString();
 #endif
 		}
+
+		private static bool Issue3702(MyClass c)
+		{
+			return c?.NullableBool.GetValueOrDefault() ?? false;
+		}
+
+		private static string Issue3702ToString(MyClass c)
+		{
+			return c?.NullableBool.GetValueOrDefault().ToString();
+		}
 	}
 }