Optionally pick the cheapest loop_vec_info

Optionally pick the cheapest loop_vec_info

This patch adds a mode in which the vectoriser tries each available
base vector mode and picks the one with the lowest cost.  The new
behaviour is selected by autovectorize_vector_modes.

The patch keeps the current behaviour of preferring a VF of
loop->simdlen over any larger or smaller VF, regardless of costs
or target preferences.

2019-11-16  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* target.h (VECT_COMPARE_COSTS): New constant.
	* target.def (autovectorize_vector_modes): Return a bitmask of flags.
	* doc/tm.texi: Regenerate.
	* targhooks.h (default_autovectorize_vector_modes): Update accordingly.
	* targhooks.c (default_autovectorize_vector_modes): Likewise.
	* config/aarch64/aarch64.c (aarch64_autovectorize_vector_modes):
	Likewise.
	* config/arc/arc.c (arc_autovectorize_vector_modes): Likewise.
	* config/arm/arm.c (arm_autovectorize_vector_modes): Likewise.
	* config/i386/i386.c (ix86_autovectorize_vector_modes): Likewise.
	* config/mips/mips.c (mips_autovectorize_vector_modes): Likewise.
	* tree-vectorizer.h (_loop_vec_info::vec_outside_cost)
	(_loop_vec_info::vec_inside_cost): New member variables.
	* tree-vect-loop.c (_loop_vec_info::_loop_vec_info): Initialize them.
	(vect_better_loop_vinfo_p, vect_joust_loop_vinfos): New functions.
	(vect_analyze_loop): When autovectorize_vector_modes returns
	VECT_COMPARE_COSTS, try vectorizing the loop with each available
	vector mode and picking the one with the lowest cost.
	(vect_estimate_min_profitable_iters): Record the computed costs
	in the loop_vec_info.

From-SVN: r278336