Improve php-syntax-propertize-extend-region efficiency

If the first pair of searches found a heredoc which spanned the entire
region, don't perform the second pair of searches, as we already know
the new region bounds.

If we do need the second pair of searches, bound the backwards search
at the original region START position.

Author: Phil Sainty

lisp/php-mode.el

@@ -1038,17 +1038,24 @@ HEREDOC-START."
       (unwind-protect
           (let (new-start new-end)
             (goto-char start)
+            ;; Consider bounding this backwards search by `beginning-of-defun'.
+            ;; (Benchmarking for a wide range of cases may be needed to decide
+            ;; whether that's an improvement, as `php-beginning-of-defun' also
+            ;; uses `re-search-backward'.)
             (when (re-search-backward php-heredoc-start-re nil t)
               (let ((maybe (point)))
                 (when (and (re-search-forward (php-heredoc-end-re (match-string 0)) nil t)
                            (> (point) start))
-                  (setq new-start maybe))))
-            (goto-char end)
-            (when (re-search-backward php-heredoc-start-re nil t)
-              (if (re-search-forward (php-heredoc-end-re (match-string 0)) nil t)
+                  (setq new-start maybe)
                   (when (> (point) end)
-                    (setq new-end (point)))
-                (setq new-end (point-max))))
+                    (setq new-end (point))))))
+            (unless new-end
+              (goto-char end)
+              (when (re-search-backward php-heredoc-start-re start t)
+                (if (re-search-forward (php-heredoc-end-re (match-string 0)) nil t)
+                    (when (> (point) end)
+                      (setq new-end (point)))
+                  (setq new-end (point-max)))))
             (when (or new-start new-end)
               (cons (or new-start start) (or new-end end))))
         ;; Cleanup