Zap OccInfo on case binders during StgCse #14895 #24233

StgCse can revive dead binders:

  case foo of dead { Foo x y -> Foo x y; ... }
  ===>
  case foo of dead { Foo x y -> dead; ... } -- dead is no longer dead

So we must zap occurrence information on case binders.

Fix #14895 and #24233