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))