belower: In push_through_perm() merge identifying the move frontier into the actual Perm moving loop.
belower: In push_through_perm() merge identifying the move frontier into the actual Perm moving loop.

* This seems to remove the non-dying check.
  But the actual moving loop already had a more strict check for any operand in the Perm's register class.
* Remove the explicit is_Phi() test.
  Phis are schedule_first, so the other part of the condition covers them, too.

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