Avoid recomputing data references in BB SLP

Avoid recomputing data references in BB SLP

If the first attempt at applying BB SLP to a region fails, the main loop
in vect_slp_bb recomputes the region's bounds and datarefs for the next
vector size.  AFAICT this isn't needed any more; we should be able
to reuse the datarefs from the first attempt instead.

2019-10-20  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-slp.c (vect_slp_analyze_bb_1): Call save_datarefs
	when processing the given datarefs for the first time and
	check_datarefs subsequently.
	(vect_slp_bb_region): New function, split out of...
	(vect_slp_bb): ...here.  Don't recompute the region bounds and
	dataref sets when retrying with a different vector size.

From-SVN: r277210