poly_int: vect_nunits_for_cost

poly_int: vect_nunits_for_cost

This patch adds a function for getting the number of elements in
a vector for cost purposes, which is always constant.  It makes
it possible for a later patch to change GET_MODE_NUNITS and
TYPE_VECTOR_SUBPARTS to a poly_int.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vectorizer.h (vect_nunits_for_cost): New function.
	* tree-vect-loop.c (vect_model_reduction_cost): Use it.
	* tree-vect-slp.c (vect_analyze_slp_cost_1): Likewise.
	(vect_analyze_slp_cost): Likewise.
	* tree-vect-stmts.c (vect_model_store_cost): Likewise.
	(vect_model_load_cost): Likewise.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r256128