Reduce the cost in miss rate computation.

Reduce the cost in miss rate computation.

2010-07-02  Changpeng Fang  <changpeng.fang@amd.com>

	* tree-ssa-loop-prefetch.c (compute_miss_rate): Rename to
	is_miss_rate_acceptable. Pull total_positions computation
	out of the loops.  Early return if miss_positions exceeds
	the acceptable threshold.
	* tree-ssa-loop-prefetch.c (prune_ref_by_group_reuse): Call
	is_miss_rate_acceptable after renaming of compute_miss_rate.

From-SVN: r161728