Fix class downcasting

[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.

Fixes #13244

[chrisbra]

Thanks. @yegappan are you fine with this?

[errael]

Downloaded and played with it. Tried a few things, no problems. Didn't really look at the implementation.

the target type may be a subclass (downcast) or superclass (upcast) of the source one

About nomenclature; there's a typo in the title (I think). Shouldn't the title be about downcasting; casting to a subclass is what failed.

[LemonBoy]

About nomenclature; there's a typo in the title (I think). Shouldn't the title be about downcasting; casting to a subclass is what failed.

Indeed, please remember to fix the typo in the commit message before merging.

[yegappan]

Thanks. @yegappan are you fine with this?

The change looks good to me. It will be good to add a disassemble test for this.

[chrisbra]

thanks!