utf8_to_uv_msgs: Use a single bit flag

This condition used a combination of two bits, which makes things a
little awkward, and isn't really needed

Author: khwilliamson

utf8.c

@@ -2092,17 +2092,13 @@ Perl_utf8_to_uv_msgs_helper_(const U8 * const s0,
               case UTF8_GOT_LONG:
                 *errors |= UTF8_GOT_LONG;
 
-                if (flags & UTF8_ALLOW_LONG) {
-
-                    /* We don't allow the actual overlong value, unless the
-                     * special extra bit is also set */
-                    if (! (flags & (   UTF8_ALLOW_LONG_AND_ITS_VALUE
-                                    & ~UTF8_ALLOW_LONG)))
-                    {
-                        uv = UNICODE_REPLACEMENT;
-                    }
+                if (! (flags & UTF8_ALLOW_LONG_AND_ITS_VALUE)) {
+                    uv = UNICODE_REPLACEMENT;
                 }
-                else {
+
+                if (! (flags & ( UTF8_ALLOW_LONG
+                                |UTF8_ALLOW_LONG_AND_ITS_VALUE)))
+                {
                     disallowed = TRUE;
 
                     if (NEED_MESSAGE(WARN_UTF8,,)) {

utf8.h

@@ -1201,7 +1201,7 @@ point's representation.
  * First one will convert the overlong to the REPLACEMENT CHARACTER; second
  * will return what the overlong evaluates to */
 #define UTF8_ALLOW_LONG                 0x2000
-#define UTF8_ALLOW_LONG_AND_ITS_VALUE   (UTF8_ALLOW_LONG|0x4000)
+#define UTF8_ALLOW_LONG_AND_ITS_VALUE   0x4000
 #define UTF8_GOT_LONG                   UTF8_ALLOW_LONG
 
 /* For back compat, these old names are misleading for overlongs and