* gc.c (rgengc_check_shady): add new WB miss checking
  on RGENGC_CHECK_MODE >= 2.

    (1) Save bitmaps before marking
    (2) Run full marking
    (3) On each traceable object,
      (a) object        was not oldgen (== newly or shady object) &&
      (b) parent object was     oldgen &&
      (c) parent object was not remembered &&
      (d) object        was not rememberd
      then, it should be WB miss.

  This idea of this checker is by Masaya Tarui <tarui@ruby-lang.org>.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41564 b2dd03c8-39d4-4d8f-98ff-823fe69b080e