fix O(n^2) length calls in compact-ir lowering step (#50756)

This can be a problem for very long function bodies.

src/julia-syntax.scm

@@ -4980,6 +4980,7 @@ f(x) = yt(x)
  (let ((code '(block))
  (locs '(list))
  (linetable '(list))
+ (linetablelen 0)
  (labltable (table))
  (ssavtable (table))
  (current-loc 0)
@@ -4992,6 +4993,7 @@ f(x) = yt(x)
  (if (and (null? (cdr linetable))
  (not (and (pair? e) (eq? (car e) 'meta))))
  (begin (set! linetable (cons (make-lineinfo name file line) linetable))
+ (set! linetablelen (+ linetablelen 1))
  (set! current-loc 1)))
  (set! code (cons e code))
  (set! i (+ i 1))
@@ -5013,13 +5015,15 @@ f(x) = yt(x)
  (make-lineinfo name current-file current-line)
  (make-lineinfo name current-file current-line (caar locstack)))
  linetable))
- (set! current-loc (- (length linetable) 1)))))
+ (set! linetablelen (+ linetablelen 1))
+ (set! current-loc linetablelen))))
  ((and (length> e 2) (eq? (car e) 'meta) (eq? (cadr e) 'push_loc))
  (set! locstack (cons (list current-loc current-line current-file) locstack))
  (set! current-file (caddr e))
  (set! current-line 0)
  (set! linetable (cons (make-lineinfo name current-file current-line current-loc) linetable))
- (set! current-loc (- (length linetable) 1)))
+ (set! linetablelen (+ linetablelen 1))
+ (set! current-loc linetablelen))
  ((and (length= e 2) (eq? (car e) 'meta) (eq? (cadr e) 'pop_loc))
  (let ((l (car locstack)))
  (set! locstack (cdr locstack))