vect: Hookize better_loop_vinfo_p

One of the things we want to do on AArch64 is compare vector loops
side-by-side and pick the best one.  For some targets, we want this
to be based on issue rates as well as the usual latency-based costs
(at least for loops with relatively high iteration counts).

The current approach to doing this is: when costing vectorisation
candidate A, try to guess what the other main candidate B will look
like and adjust A's latency-based cost up or down based on the likely
difference between A and B's issue rates.  This effectively means
that we try to cost parts of B at the same time as A, without actually
being able to see B.

This is needlessly indirect and complex.  It was a compromise due
to the code being added (too) late in the GCC 11 cycle, so that
target-independent changes weren't possible.

The target-independent code already compares two candidate loop_vec_infos
side-by-side, so that information about A and B above are available
directly.  This patch creates a way for targets to hook into this
comparison.

The AArch64 code can therefore hook into better_main_loop_than_p to
compare issue rates.  If the issue rate comparison isn't decisive,
the code can fall back to the normal latency-based comparison instead.

gcc/
	* tree-vectorizer.h (vector_costs::better_main_loop_than_p)
	(vector_costs::better_epilogue_loop_than_p)
	(vector_costs::compare_inside_loop_cost)
	(vector_costs::compare_outside_loop_cost): Likewise.
	* tree-vectorizer.c (vector_costs::better_main_loop_than_p)
	(vector_costs::better_epilogue_loop_than_p)
	(vector_costs::compare_inside_loop_cost)
	(vector_costs::compare_outside_loop_cost): New functions,
	containing code moved from...
	* tree-vect-loop.c (vect_better_loop_vinfo_p): ...here.

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