vect, aarch64: Extend SVE vs Advanced SIMD costing decisions in vect_better_loop_vinfo_p

While experimenting with some backend costs for Advanced SIMD and SVE I
hit many cases where GCC would pick SVE for VLA auto-vectorisation even when
the backend very clearly presented cheaper costs for Advanced SIMD.
For a simple float addition loop the SVE costs were:

vec.c:9:21: note:  Cost model analysis:
  Vector inside of loop cost: 28
  Vector prologue cost: 2
  Vector epilogue cost: 0
  Scalar iteration cost: 10
  Scalar outside cost: 0
  Vector outside cost: 2
  prologue iterations: 0
  epilogue iterations: 0
  Minimum number of vector iterations: 1
  Calculated minimum iters for profitability: 4

and for Advanced SIMD (Neon) they're:

vec.c:9:21: note:  Cost model analysis:
  Vector inside of loop cost: 11
  Vector prologue cost: 0
  Vector epilogue cost: 0
  Scalar iteration cost: 10
  Scalar outside cost: 0
  Vector outside cost: 0
  prologue iterations: 0
  epilogue iterations: 0
  Calculated minimum iters for profitability: 0
vec.c:9:21: note:    Runtime profitability threshold = 4

yet the SVE one was always picked. With guidance from Richard this seems
to be due to the vinfo comparisons in vect_better_loop_vinfo_p, in
particular the part with the big comment explaining the
estimated_rel_new * 2 <= estimated_rel_old heuristic.

This patch extends the comparisons by introducing a three-way estimate
kind for poly_int values that the backend can distinguish.
This allows vect_better_loop_vinfo_p to ask for minimum, maximum and
likely estimates and pick Advanced SIMD overs SVE when it is clearly cheaper.

gcc/
	* target.h (enum poly_value_estimate_kind): Define.
	(estimated_poly_value): Take an estimate kind argument.
	* target.def (estimated_poly_value): Update definition for the
	above.
	* doc/tm.texi: Regenerate.
	* targhooks.c (estimated_poly_value): Update prototype.
	* tree-vect-loop.c (vect_better_loop_vinfo_p): Use min, max and
	likely estimates of VF to pick between vinfos.
	* config/aarch64/aarch64.c (aarch64_cmp_autovec_modes): Use
	estimated_poly_value instead of aarch64_estimated_poly_value.
	(aarch64_estimated_poly_value): Take a kind argument and handle
	it.

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