Refactor and comment JvmCodegenUtil#couldUseDirectAccessToProperty()

Logic hasn't changed except the deleted isTopLevelDeclaration() call which
always returned false in this case because we checked that it's a class
property earlier
Use direct field access to trivial class properties

 #KT-3845 Fixed
Don't generate unnecessary accessors for private class properties
Access private class property via field in Evaluate Expression
Minor, delete unused method in JvmCodegenUtil