Only test STMT_VINFO_STRIDED_P for the first statement in a group

Only test STMT_VINFO_STRIDED_P for the first statement in a group

get_load_store_type & co were testing STMT_VINFO_STRIDED_P on individual
statements in a group instead of the first.  This has no effect on
its own, but is needed by a later patch.

2018-08-22  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vect-stmts.c (get_group_load_store_type)
	(get_load_store_type): Only test STMT_VINFO_STRIDED_P for the
	first statement in a group.

From-SVN: r263771