Add an alternative vector loop iv mechanism

Normally we adjust the vector loop so that it iterates:

   (original number of scalar iterations - number of peels) / VF

times, enforcing this using an IV that starts at zero and increments
by one each iteration.  However, dividing by VF would be expensive
for variable VF, so this patch adds an alternative in which the IV
increments by VF each iteration instead.  We then need to take care
to handle possible overflow in the IV.

The new mechanism isn't used yet; a later patch replaces the
"if (1)" with a check for variable VF.

2018-01-03  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vect-loop-manip.c: Include gimple-fold.h.
	(slpeel_make_loop_iterate_ntimes): Add step, final_iv and
	niters_maybe_zero parameters.  Handle other cases besides a step of 1.
	(vect_gen_vector_loop_niters): Add a step_vector_ptr parameter.
	Add a path that uses a step of VF instead of 1, but disable it
	for now.
	(vect_do_peeling): Add step_vector, niters_vector_mult_vf_var
	and niters_no_overflow parameters.  Update calls to
	slpeel_make_loop_iterate_ntimes and vect_gen_vector_loop_niters.
	Create a new SSA name if the latter choses to use a ste other
	than zero, and return it via niters_vector_mult_vf_var.
	* tree-vect-loop.c (vect_transform_loop): Update calls to
	vect_do_peeling, vect_gen_vector_loop_niters and
	slpeel_make_loop_iterate_ntimes.
	* tree-vectorizer.h (slpeel_make_loop_iterate_ntimes, vect_do_peeling)
	(vect_gen_vector_loop_niters): Update declarations after above changes.

From-SVN: r256124