i386: Add clear_ratio to processor_costs

i386.h has

 #define CLEAR_RATIO(speed) ((speed) ? MIN (6, ix86_cost->move_ratio) : 2)

It is impossible to have CLEAR_RATIO > 6.  This patch adds clear_ratio
to processor_costs, sets it to the minimum of 6 and move_ratio in all
cost models and defines CLEAR_RATIO with clear_ratio.

	* config/i386/i386.h (processor_costs): Add clear_ratio.
	(CLEAR_RATIO): Remove MIN and use ix86_cost->clear_ratio.
	* config/i386/x86-tune-costs.h: Set clear_ratio to the minimum
	of 6 and move_ratio in all cost models.

From-SVN: r277114