A later patch makes the AArch64 port add four entries to
autovectorize_vector_modes.  Each entry describes a different
vector mode assignment for vector code that mixes 8-bit, 16-bit,
32-bit and 64-bit elements.  But if (as usual) the vector code has
fewer element sizes than that, we could end up trying the same
combination of vector modes multiple times.  This patch adds a
check to prevent that.

2019-11-14  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* tree-vectorizer.h (vec_info::mode_set): New typedef.
	(vec_info::used_vector_mode): New member variable.
	(vect_chooses_same_modes_p): Declare.
	* tree-vect-stmts.c (get_vectype_for_scalar_type): Record each
	chosen vector mode in vec_info::used_vector_mode.
	(vect_chooses_same_modes_p): New function.
	* tree-vect-loop.c (vect_analyze_loop): Use it to avoid trying
	the same vector statements multiple times.
	* tree-vect-slp.c (vect_slp_bb_region): Likewise.

From-SVN: r278242