re_intuit_start(): reduce scope of /^...$/m test

Intuit has a quick reject test for a fixed pattern that is anchored at
both ends. For example, with the pattern /^abcd$/, only the exact strings
"abcd" or "abcd\n" will match; anything else, and the match immediately
fails.

A fix for [perl #115242] correctly made intuit skip the test in the
presence of //m, since in this case the $ doesn't necessarily correspond
to the end of the string.

However, the fix was too wide in scope; it caused //m patterns to skip
searching for a known string anchored just at the start, as well as one
anchored at both ends.

With this commit, the following code now runs in a few milliseconds rather
than a few seconds on my machine:

    $s = "abcdefg" x 1_000_000;
    $s =~ /(?-m:^)abcX?fg/m for 1..100;

1 files changed, 2 insertions, 5 deletions

diff --git a/regexec.c b/regexec.c
index 54400f17ac..e3ce022528 100644
--- a/regexec.c
+++ b/regexec.c
@@ -783,10 +783,7 @@ Perl_re_intuit_start(pTHX_
              * at position pos()-4+1, which lines up with the "a" */
 
 	    if (prog->check_offset_min == prog->check_offset_max
-                && !(prog->intflags & PREGf_CANY_SEEN)
-                && ! multiline)   /* /m can cause \n's to match that aren't
-                                     accounted for in the string max length.
-                                     See [perl #115242] */
+                && !(prog->intflags & PREGf_CANY_SEEN))
             {
 	        /* Substring at constant offset from beg-of-str... */
 	        SSize_t slen = SvCUR(check);
@@ -798,7 +795,7 @@ Perl_re_intuit_start(pTHX_
                     "  Looking for check substr at fixed offset %"IVdf"...\n",
                     (IV)prog->check_offset_min));
 
-	        if (SvTAIL(check)) {
+	        if (SvTAIL(check) && !multiline) {
                     /* In this case, the regex is anchored at the end too,
                      * so the lengths must match exactly, give or take a \n.
 		     * NB: slen >= 1 since the last char of check is \n */