regexes: make scanning for ANYOF faster

Given a character class of random chars (like [acgt] say, rather than
predefined ones like [\d], say), speed up the code in:

1) S_find_byclass(), which scans for the first char in the string that's
   in that class (e.g. /[acgt]...../),
2) S_regrepeat() which scans past all chars that are in that class
   (e.g. /....[acgt]+..../)

by hoisting an unchanging test outside the main while loop. So this:

    while (s < end) {
        if (ANYOF_FLAGS(node))
            match = reginclass(*s, ...);
        else
            match = ANYOF_BITMAP_TEST(*s, ...);
        ...
    }

becomes this:

    if (ANYOF_FLAGS(node)) {
        while (s < end) {
            match = reginclass(*s, ...);
            ...
        }
    else
        while (s < end) {
            match = ANYOF_BITMAP_TEST(*s, ...);
            ...
        }
    }

The average of the 3 tests added to t/perf/benchmarks by this commit show
this change (raw numbers, lower better):

         before    after
       -------- --------
    Ir   3294.0   2763.0
    Dr    900.7    802.3
    Dw    356.0    390.0
  COND    569.0    436.7
   IND     11.0     11.0

COND_m      1.2      2.0
 IND_m      7.3      7.3

1 files changed, 12 insertions, 3 deletions

diff --git a/regexec.c b/regexec.c
index 013ccc54a8..f6f293d56e 100644
--- a/regexec.c
+++ b/regexec.c
@@ -1881,8 +1881,11 @@ S_find_byclass(pTHX_ regexp * prog, const regnode *c, char *s,
             REXEC_FBC_UTF8_CLASS_SCAN(
                       reginclass(prog, c, (U8*)s, (U8*) strend, utf8_target));
         }
+        else if (ANYOF_FLAGS(c)) {
+            REXEC_FBC_CLASS_SCAN(reginclass(prog,c, (U8*)s, (U8*)s+1, 0));
+        }
         else {
-            REXEC_FBC_CLASS_SCAN(REGINCLASS(prog, c, (U8*)s, 0));
+            REXEC_FBC_CLASS_SCAN(ANYOF_BITMAP_TEST(c, *((U8*)s)));
         }
         break;
 
@@ -8892,8 +8895,14 @@ S_regrepeat(pTHX_ regexp *prog, char **startposp, const regnode *p,
 		scan += UTF8SKIP(scan);
 		hardcount++;
 	    }
-	} else {
-	    while (scan < loceol && REGINCLASS(prog, p, (U8*)scan, 0))
+	}
+        else if (ANYOF_FLAGS(p)) {
+	    while (scan < loceol
+                    && reginclass(prog, p, (U8*)scan, (U8*)scan+1, 0))
+		scan++;
+        }
+        else {
+	    while (scan < loceol && ANYOF_BITMAP_TEST(p, *((U8*)scan)))
 		scan++;
 	}
 	break;