refactor SLP constant insertion and provde entry insert helper

refactor SLP constant insertion and provde entry insert helper

This provides helpers to insert stmts on region entry abstracted
from loop/basic-block split out from vec_init_vector and used
from the SLP constant code generation path.  The SLP constant
code generation path is also changed to avoid needless SSA
copying since we can store VECTOR_CSTs directly in the vectorized
defs array, improving the IL from the vectorizer.

2020-07-03  Richard Biener  <rguenther@suse.de>

	* tree-vectorizer.h (vec_info::insert_on_entry): New.
	(vec_info::insert_seq_on_entry): Likewise.
	* tree-vectorizer.c (vec_info::insert_on_entry): Implement.
	(vec_info::insert_seq_on_entry): Likewise.
	* tree-vect-stmts.c (vect_init_vector_1): Use
	vec_info::insert_on_entry.
	(vect_finish_stmt_generation): Set modified bit after
	adjusting VUSE.
	* tree-vect-slp.c (vect_create_constant_vectors): Simplify
	by using vec_info::insert_seq_on_entry and bypassing
	vec_init_vector.
	(vect_schedule_slp_instance): Deal with all-constant
	children later.