merge revision(s) ce7a0534,1cbb1f17: [Backport #16695]

    Adjust sp for `x = false; y = (return until x unless x)` [Bug #16695]

    test/ruby/test_syntax.rb: avoid "warning: assigned but unused
     variable"


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67925 b2dd03c8-39d4-4d8f-98ff-823fe69b080e