This patch speeds up the ifcvt_memrefs_wont_trap computation by
caching the results of the computations in the data references ->aux
fields.

	* tree-if-conv.c (struct ifc_dr): New.
	(IFC_DR): New.
	(DR_WRITTEN_AT_LEAST_ONCE): New.
	(DR_RW_UNCONDITIONALLY): New.
	(memref_read_or_written_unconditionally): Use the cached values
	when possible.
	(write_memref_written_at_least_once): Same.
	(if_convertible_loop_p): Initialize and free DR->aux fields.

From-SVN: r163532