Fix regexp performance regression for patterns starting with s/k

Commit 981ee02 ("Fix performance problem with /k/i and /s/i") was
merged for Ruby 4.0 to enable partial Boyer-Moore optimization for
patterns containing 's' or 'k' by using the prefix before those
characters.

However, when 's' or 'k' appears at the start of a pattern (no usable
prefix), set_bm_skip() returns 0 and the code returned early without
setting any optimization mode, leaving reg->optimize at
ONIG_OPTIMIZE_NONE. This caused up to 30x slowdown for patterns like
/slackware/i when matched against strings with non-ASCII characters.

This patch keeps the improvement from 981ee02 for patterns with
3+ char prefix, while fixing the regression by falling back to
ONIG_OPTIMIZE_EXACT_IC with the full pattern when the usable prefix
is less than 3 characters.

Before: /\bslackware\b/i with non-ASCII string: 2.24 us/op
After:  /\bslackware\b/i with non-ASCII string: 0.70 us/op (3.2x faster)

[Bug #21824]

Author: khasinski

regcomp.c

@@ -5264,18 +5264,24 @@ set_optimize_exact_info(regex_t* reg, OptExactInfo* e)
 
   if (e->ignore_case > 0) {
     if (e->len >= 3 || (e->len >= 2 && allow_reverse)) {
+      int orig_len = e->len;
       e->len = set_bm_skip(reg->exact, reg->exact_end, reg,
                            reg->map, 1);
-      reg->exact_end = reg->exact + e->len;
       if (e->len >= 3) {
+        reg->exact_end = reg->exact + e->len;
         reg->optimize = (allow_reverse != 0
                          ? ONIG_OPTIMIZE_EXACT_BM_IC : ONIG_OPTIMIZE_EXACT_BM_NOT_REV_IC);
       }
-      else if (e->len > 0) {
+      else {
+        /* Even if BM skip table can't be built (e.g., pattern starts with
+           's' or 'k' which have multi-byte case fold variants), we should
+           still use EXACT_IC optimization with the original pattern.
+           Without this fallback, patterns like /slackware/i have no
+           optimization at all, causing severe performance regression
+           especially with non-ASCII strings. See [Bug #21824] */
+        e->len = orig_len;  /* Restore original length for EXACT_IC */
         reg->optimize = ONIG_OPTIMIZE_EXACT_IC;
       }
-      else
-        return 0;
     }
     else {
       reg->optimize = ONIG_OPTIMIZE_EXACT_IC;