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merge revision(s) 53039: [Backport #11804]

    * insns.def (opt_case_dispatch): avoid converting Infinity

    * test/ruby/test_optimization.rb (test_opt_case_dispatch_inf): new
      [ruby-dev:49423] [Bug #11804]'

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_1@53147 b2dd03c8-39d4-4d8f-98ff-823fe69b080e