Move code out of vect_slp_analyze_bb_1

Move code out of vect_slp_analyze_bb_1

After the previous patch, it seems more natural to apply the
PARAM_SLP_MAX_INSNS_IN_BB threshold as soon as we know what
the region is, rather than delaying it to vect_slp_analyze_bb_1.
(But rather than carve out the biggest region possible and then
reject it, wouldn't it be better to stop when the region gets
too big, to at least give us a chance of vectorising something?)

It also seems more natural for vect_slp_bb_region to create the
bb_vec_info itself rather than (a) having to pass bits of data down
for the initialisation and (b) forcing vect_slp_analyze_bb_1 to free
on every failure return.

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

gcc/
	* tree-vect-slp.c (vect_slp_analyze_bb_1): Take a bb_vec_info
	and return a boolean success value.  Move the allocation and
	initialization of the bb_vec_info to...
	(vect_slp_bb_region): ...here.  Update call accordingly.
	(vect_slp_bb): Apply PARAM_SLP_MAX_INSNS_IN_BB here rather
	than in vect_slp_analyze_bb_1.

From-SVN: r277211