[22/46] Make DR_GROUP_SAME_DR_STMT a stmt_vec_info

[22/46] Make DR_GROUP_SAME_DR_STMT a stmt_vec_info

This patch changes STMT_VINFO_SAME_DR_STMT from a gimple stmt to a
stmt_vec_info.

2018-07-31  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (_stmt_vec_info::same_dr_stmt): Change from
	a gimple stmt to a stmt_vec_info.
	* tree-vect-stmts.c (vectorizable_load): Update accordingly.

From-SVN: r263137