ifcvt/optabs: Allow using a CC comparison for emit_conditional_move.

Currently we only ever call emit_conditional_move with the comparison
(as well as its comparands) we got from the jump.  Thus, backends are
going to emit a CC comparison for every conditional move that is being
generated instead of re-using the existing CC.
This, combined with emitting temporaries for each conditional move,
causes sky-high costs for conditional moves.

This patch allows to re-use a CC so the costing situation is improved a
bit.

gcc/ChangeLog:

	* rtl.h (struct rtx_comparison): New struct that holds an rtx
	comparison.
	* config/rs6000/rs6000.cc (rs6000_emit_minmax): Use struct instead of
	single parameters.
	(rs6000_emit_swsqrt): Likewise.
	* expmed.cc (expand_sdiv_pow2): Likewise.
	(emit_store_flag): Likewise.
	* expr.cc (expand_cond_expr_using_cmove): Likewise.
	(expand_expr_real_2): Likewise.
	* ifcvt.cc (noce_emit_cmove): Add compare and reversed compare
	parameters.
	* optabs.cc (emit_conditional_move_1): New function.
	(expand_doubleword_shift_condmove): Use struct.
	(emit_conditional_move): Use struct and allow to call directly
	without going through preparation steps.
	* optabs.h (emit_conditional_move): Use struct.