Fix use after free in vector_size change

Fix use after free in vector_size change

r277235 was a bit too mechanical and ended up introducing use
after free bugs in both loop and SLP vectorisation.

2019-10-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-slp.c (vect_slp_bb_region): Check whether
	autodetected_vector_size rather than vector_size is zero.
	* tree-vect-loop.c (vect_analyze_loop): Likewise.
	Set autodetected_vector_size immediately after calling
	vect_analyze_loop_2.  Check for a fatal error before advancing
	next_size.

From-SVN: r277282