[hl] fix return in generic with void parameter

[yuxiaomao]

For the following repro:

class Base<T> {
	public function new() {}
	public function call() : T {
		return null;
	}
	public function call2() : T {
		return null;
	}
}

class Sub extends Base<Void> {
	var count = 0;
	override function call() : Void {
		count++;
		if( count == 1 )
			return;
		count++;
	}
	override function call2() : Void {
	}
}

function main() {
	var i : Base<Void> = new Sub();
	i.call();
	i.call2();
}

call fail at hl-check:

 16 |    return;
    |    ^^^^^^
    | Check failure at fun@27 @6 - Register 3(void) should be dyn and not void

and call2 fail as fatal error (probably caused by the 0 opcode function of call2)

Fatal error: exception Invalid_argument("index out of bounds")
Raised by primitive operation at Hlopt.code_graph.op in file "src/generators/hlopt.ml" (inlined), line 463, characters 16-30
Called from Hlopt.code_graph.make_block.loop in file "src/generators/hlopt.ml", line 497, characters 27-33
Called from Hlopt.code_graph.make_block in file "src/generators/hlopt.ml", line 534, characters 3-11
Called from Hlopt.code_graph in file "src/generators/hlopt.ml", line 537, characters 13-28
Called from Hlopt._optimize in file "src/generators/hlopt.ml", line 771, characters 24-36
Called from Hlopt.optimize in file "src/generators/hlopt.ml", line 1113, characters 13-24
Called from Genhl.generate.(fun) in file "src/generators/genhl.ml", line 4399, characters 21-78

This PR try to adresse them by:

• Generate r:dyn (or whatever ctx.m.mret is), ONull r, ORet r instead of r:void, ORet r for the middle return;.
• Treat an empty block as not having a final return (I think the name have_final_jump is misleading, maybe it's something like need_final_return)