an endless `loop` block lowers to the single ir loop kind, with a
synthetic `true` condition. exiting the loop requires an explicit
`break` from inside the body.

<<<

main = fun () {
    var count int = 0
    loop {
        count++
        if count == 5 {
            break
        }
    }
}

>>>

(unit
	(types
		(type 0 int primitive)
		(type 1 uint primitive)
		(type 2 bool primitive)
		(type 3 string primitive)
		(type 4 float primitive)
		(type 5 byte primitive)
		(type 6 ascii primitive)
		(type 7 void primitive))
	(functions
		(function 0 main main (returns (ref void)) (block
			(declaration count (ref int) (initializer (expr 0)))
			(loop (condition (expr true)) (block
				(expression-stmt (expr (postfix-++ (expr (name count)))))
				(conditional (when (expr (binary == (expr (name count)) (expr 5)))) (block
					(break)
				))
			))
		)))
	(emission_order 0 1 2 3 4 5 6 7))