* regcomp.c, regexec.c: Optimization should be disabled not only for
  /(?<=x).*/ but also for /(?!x).*/.
  (merge r47598 partially. [Bug #9728])


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@47976 b2dd03c8-39d4-4d8f-98ff-823fe69b080e