Fix a slowdown in footnote parsing.

* lisp/org-footnote.el (org-footnote-in-valid-context-p): Fix slowdown.

Apply de Morgan’s law and reorder clauses from (apparently) quickest to
slowest.  See <http://mid.gmane.org/loom.20151204T081351-244@post.gmane.org>.

lisp/org-footnote.el

@@ -200,21 +200,24 @@ extracted will be filled again."
 (defun org-footnote-in-valid-context-p ()
   "Is point in a context where footnotes are allowed?"
   (save-match-data
-    (not (or (org-at-comment-p)
-	     (org-inside-LaTeX-fragment-p)
-	     ;; Avoid literal example.
-	     (org-in-verbatim-emphasis)
-	     (save-excursion
-	       (beginning-of-line)
-	       (looking-at "[ \t]*:[ \t]+"))
-	     ;; Avoid cited text and headers in message-mode.
-	     (and (derived-mode-p 'message-mode)
-		  (or (save-excursion
-			(beginning-of-line)
-			(looking-at message-cite-prefix-regexp))
-		      (message-point-in-header-p)))
-	     ;; Avoid forbidden blocks.
-	     (org-in-block-p org-footnote-forbidden-blocks)))))
+    (and
+     (not (org-at-comment-p))
+     (not (org-in-verbatim-emphasis))
+     ;; Avoid forbidden blocks.
+     (not (org-in-block-p org-footnote-forbidden-blocks))
+     ;; Avoid literal example.
+     (not (save-excursion
+	    (beginning-of-line)
+	    (looking-at "[ \t]*:[ \t]+")))
+     ;; The latex fragment check seems expensive, so save it for last.
+     ;; See <http://mid.gmane.org/loom.20151204T081351-244@post.gmane.org>.
+     (not (org-inside-LaTeX-fragment-p))
+     ;; Avoid cited text and headers in message-mode.
+     (not (and (derived-mode-p 'message-mode)
+	       (or (save-excursion
+		     (beginning-of-line)
+		     (looking-at message-cite-prefix-regexp))
+		   (message-point-in-header-p)))))))
 
 (defun org-footnote-at-reference-p ()
   "Is the cursor at a footnote reference?