patch 9.1.0523: Vim9: cannot downcast an object

patch 9.1.0523: Vim9: cannot downcast an object

Problem:  Vim9: cannot downcast an object (Ernie Rael)
Solution: Fix class downcasting issue (LemonBoy).

When casting an object from one class to another the target type may be
a subclass (downcast) or superclass (upcast) of the source one.
Upcasts require a runtime type check to be emitted.

Add a disassembly test.

fixes: #13244
closes: #15079

Signed-off-by: LemonBoy <thatlemon@gmail.com>
Signed-off-by: Yegappan Lakshmanan <yegappan@yahoo.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>