swift234

Teach allocbox_to_stack to be a bit smarter when removing dead instructions:
have it remove trivially dead operands of the dead instructions.  On the
same testcase as last time, this eliminates a dead metatype, getting us to:

sil @_T1t1fFT_T_ : $@thin () -> () {
bb0:
  %0 = tuple ()
  %1 = alloc_stack $Int64  // var a               // users: %7, %6
  %2 = metatype $Int64.metatype
  %3 = module #Builtin
  %4 = integer_literal $Builtin.Int64, 1          // user: %5
  %5 = struct $Int64 (%4 : $Builtin.Int64)        // user: %6
  store %5 to %1#1 : $*Int64
  dealloc_stack %1#0 : $*@local_storage Int64
  %8 = tuple ()                                   // user: %9
  return %8 : $()
}



Swift SVN r9306