* insns.def (opt_regexpmatch2): Check String#=~ hasn't overridden
  before calling rb_reg_match().

* test/ruby/test_string.rb: Test for above.

* vm.c (vm_init_redefined_flag): Add BOP flag for String#=~

[ruby-core:57385] [Bug #8953]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43052 b2dd03c8-39d4-4d8f-98ff-823fe69b080e