This patch adds a new helper function for permanently removing a
statement and its associated stmt_vec_info.

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

gcc/
	* tree-vectorizer.h (vec_info::remove_stmt): Declare.
	* tree-vectorizer.c (vec_info::remove_stmt): New function.
	* tree-vect-loop-manip.c (vect_set_loop_condition): Use it.
	* tree-vect-loop.c (vect_transform_loop): Likewise.
	* tree-vect-slp.c (vect_schedule_slp): Likewise.
	* tree-vect-stmts.c (vect_remove_stores): Likewise.

From-SVN: r263156