A pattern's PATTERN_DEF_SEQ was attached to both the original statement
and the main pattern statement, which made it harder to update later.
This patch attaches it to just the original statement.  In practice,
anything that cared had ready access to both.

2018-06-20  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (_stmt_vec_info): Note above pattern_def_seq
	that the sequence is attached to the original statement rather
	than the pattern statement.
	* tree-vect-loop.c (vect_determine_vf_for_stmt): Take the
	PATTERN_DEF_SEQ from the original statement rather than
	the main pattern statement.
	* tree-vect-stmts.c (free_stmt_vec_info): Likewise.
	* tree-vect-patterns.c (vect_recog_dot_prod_pattern): Likewise.
	(vect_mark_pattern_stmts): Don't copy the PATTERN_DEF_SEQ.

From-SVN: r261785