Switch regexp strategy for regexps that are just plain

Switch regexp strategy for regexps that are just plain
strings with a small alphabet.  We already have code
that handles these regexps well, we were just not always
activating it.
Committed: https://code.google.com/p/v8/source/detail?r=11218

[ulan, 2012-04-03 11:43:13]

LGTM

https://chromiumcodereview.appspot.com/9959096/diff/1/src/jsregexp.cc
File src/jsregexp.cc (right):

https://chromiumcodereview.appspot.com/9959096/diff/1/src/jsregexp.cc#newcode132
src/jsregexp.cc:132: if (different * 3 > length) return false;
A comment explaining '3' would be helpful.