utf8.c: Move most important conditional to be first

It turns out that the information generated in this block is only needed
if the final conditional in this complicated group of them is true,
which checks if the caller wants anything special for certain classes of
code points.  Because that final condition is subsidiary, the block was
getting executed just to be thrown away.

Author: khwilliamson

utf8.c

@@ -1591,10 +1591,12 @@ Perl__utf8n_to_uvchr_msgs_helper(const U8 *s,
     }
 
     /* Here, we have found all the possible problems, except for when the input
-     * is for a problematic code point not allowed by the input parameters. */
-
+     * is for a problematic code point not allowed by the input parameters.
+     * Check now for those parameters */
+    if (   (flags & ( UTF8_DISALLOW_ILLEGAL_INTERCHANGE
+                     |UTF8_WARN_ILLEGAL_INTERCHANGE))
                                 /* uv is valid for overlongs */
-    if (   (   (      LIKELY(! (possible_problems & ~UTF8_GOT_LONG))
+        && (   (      LIKELY(! (possible_problems & ~UTF8_GOT_LONG))
                    && isUNICODE_POSSIBLY_PROBLEMATIC(uv))
             || (   UNLIKELY(possible_problems)
 
@@ -1604,9 +1606,7 @@ Perl__utf8n_to_uvchr_msgs_helper(const U8 *s,
                            * code */
                 && LIKELY(! (possible_problems & UTF8_GOT_OVERFLOW))
                 && (   isUTF8_POSSIBLY_PROBLEMATIC(*adjusted_s0)
-                    || UNLIKELY(UTF8_IS_PERL_EXTENDED(s0)))))
-        && ((flags & ( UTF8_DISALLOW_ILLEGAL_INTERCHANGE
-                      |UTF8_WARN_ILLEGAL_INTERCHANGE))))
+                    || UNLIKELY(UTF8_IS_PERL_EXTENDED(s0))))))
     {
         /* If there were no malformations, or the only malformation is an
          * overlong, 'uv' is valid */