utf8n_to_uv_msgs: Avoid unnecessary work

The outermost block here is executed when any of three types of
problematic Unicode code points is encountered, and the caller has
indicated special handling of at least one of those types.  Before this
commit, we set a flag to later look to see if what was encountered
matched the type the caller specified.  This commit changes to do that
looking at the point where the flag had been set, and only sets the flag
if necessary.  This may completely avoid the later work, which has
set-up overhead, and this will make future commits simpler.

utf8.c

@@ -1858,13 +1858,19 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
              * loop just above. */
             if (isUNICODE_POSSIBLY_PROBLEMATIC(uv)) {
                 if (UNLIKELY(UNICODE_IS_SURROGATE(uv))) {
-                    possible_problems |= UTF8_GOT_SURROGATE;
+                    if (flags & (UTF8_DISALLOW_SURROGATE|UTF8_WARN_SURROGATE)) {
+                        possible_problems |= UTF8_GOT_SURROGATE;
+                    }
                 }
                 else if (UNLIKELY(UNICODE_IS_SUPER(uv))) {
-                    possible_problems |= UTF8_GOT_SUPER;
+                    if (flags & (UTF8_DISALLOW_SUPER|UTF8_WARN_SUPER)) {
+                        possible_problems |= UTF8_GOT_SUPER;
+                    }
                 }
                 else if (UNLIKELY(UNICODE_IS_NONCHAR(uv))) {
-                    possible_problems |= UTF8_GOT_NONCHAR;
+                    if (flags & (UTF8_DISALLOW_NONCHAR|UTF8_WARN_NONCHAR)) {
+                        possible_problems |= UTF8_GOT_NONCHAR;
+                    }
                 }
             }
         }
@@ -2039,6 +2045,10 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
 
               case UTF8_GOT_SURROGATE:
 
+                /* Code earlier in this function has set things up so we don't
+                 * get here unless at least one of the two top-level 'if's in
+                 * this case are true */
+
                 if (flags & UTF8_WARN_SURROGATE) {
                     *errors |= UTF8_GOT_SURROGATE;
 
@@ -2068,6 +2078,10 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
 
               case UTF8_GOT_NONCHAR:
 
+                /* Code earlier in this function has set things up so we don't
+                 * get here unless at least one of the two top-level 'if's in
+                 * this case are true */
+
                 if (flags & UTF8_WARN_NONCHAR) {
                     *errors |= UTF8_GOT_NONCHAR;
 
@@ -2202,6 +2216,11 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
 
               case UTF8_GOT_SUPER:
 
+                /* We get here when the input is for an above Unicode code
+                 * point, but it does not use Perl extended UTF-8, and the
+                 * caller has indicated that these are to be disallowed and/or
+                 * warned about */
+
                 if (flags & UTF8_WARN_SUPER) {
                     *errors |= UTF8_GOT_SUPER;