Allocate a stack only if there's a need

If there's no OP_PUSH there's no need to eagerly preallocate it.

    (loop [n 0] (if (= n 10000000) n (recur (inc n))))
    ;; Before  1.97s
    ;; After   0.96s

    (loop [n 0] (if (= n 200000000) n (recur (+ 1 n))))
    ;; Before  2.51s
    ;; After   2.33s

    (last (range 1000000))
    ;; Before  2.52s
    ;; After   2.08s