vect: Refactor peel_iters_{pro,epi}logue cost modeling

vect: Refactor peel_iters_{pro,epi}logue cost modeling

This patch is to refactor the existing peel_iters_prologue and
peel_iters_epilogue cost model handlings, by following the structure
below suggested by Richard Sandiford:

  - calculate peel_iters_prologue
  - calculate peel_iters_epilogue
  - add costs associated with peel_iters_prologue
  - add costs associated with peel_iters_epilogue
  - add costs related to branch taken/not_taken.

Bootstrapped/regtested on aarch64-linux-gnu.

gcc/ChangeLog:

	* tree-vect-loop.c (vect_get_known_peeling_cost): Factor out some code
	to determine peel_iters_epilogue to...
	(vect_get_peel_iters_epilogue): ...this new function.
	(vect_estimate_min_profitable_iters): Refactor cost calculation on
	peel_iters_prologue and peel_iters_epilogue.