rtl-ssa: Don't cost no-op moves

rtl-ssa: Don't cost no-op moves

No-op moves are given the code NOOP_MOVE_INSN_CODE if we plan
to delete them later.  Such insns shouldn't be costed, partly
because they're going to disappear, and partly because targets
won't recognise the insn code.

gcc/
	* rtl-ssa/changes.cc (rtl_ssa::changes_are_worthwhile): Don't
	cost no-op moves.
	* rtl-ssa/insns.cc (insn_info::calculate_cost): Likewise.