Protect against min_profitable_iters going negative

Protect against min_profitable_iters going negative

We had:

      if (vec_outside_cost <= 0)
        min_profitable_iters = 0;
      else
        {
	  min_profitable_iters = ((vec_outside_cost - scalar_outside_cost)
				  * assumed_vf
				  - vec_inside_cost * peel_iters_prologue
				  - vec_inside_cost * peel_iters_epilogue)
				 / ((scalar_single_iter_cost * assumed_vf)
				    - vec_inside_cost);

which can lead to negative min_profitable_iters when the *_outside_costs
are the same and peel_iters_epilogue is nonzero (e.g. if we're peeling
for gaps).

This is tested as part of the patch that adds support for fully-predicated
loops.

2018-01-13  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-loop.c (vect_estimate_min_profitable_iters): Make sure
	min_profitable_iters doesn't go negative.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r256621