ifcvt: Try re-using CC for conditional moves.

Following up on the previous patch, this patch makes
noce_convert_multiple emit two cmov sequences:  The same one as before
and a second one that tries to re-use the existing CC.  Then their costs
are compared and the cheaper one is selected.

gcc/ChangeLog:

	* ifcvt.cc (cond_exec_get_condition): New parameter to allow getting the
	reversed comparison.
	(try_emit_cmove_seq): New function to facilitate creating a cmov
	sequence.
	(noce_convert_multiple_sets): Create two sequences and use the less
	expensive one.