[SLP] SLP vectorization: vectorize vector constructors

[SLP] SLP vectorization: vectorize vector constructors

gcc/ChangeLog:
2019-11-04  Joel Hutton  <Joel.Hutton@arm.com>

	* expr.c (store_constructor): Modify to handle single element vectors.
	* tree-vect-slp.c (vect_analyze_slp_instance): Add case for vector
	constructors.
	(vect_slp_check_for_constructors): New function.
	(vect_slp_analyze_bb_1): Call new function to check for vector
	constructors.
	(vectorize_slp_instance_root_stmt): New function.
	(vect_schedule_slp): Call new function to vectorize root stmt of vector
	constructors.
	* tree-vectorizer.h (SLP_INSTANCE_ROOT_STMT): New field.

gcc/testsuite/ChangeLog:
2019-11-04  Joel Hutton  <Joel.Hutton@arm.com>

	* gcc.dg/vect/bb-slp-40.c: New test.
	* gcc.dg/vect/bb-slp-41.c: New test.

From-SVN: r277784