poly_int: two-operation SLP

This patch makes two-operation SLP handle but reject variable-length
vectors.  Adding support for this is a post-GCC8 thing.

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

gcc/
	* tree-vect-slp.c (vect_build_slp_tree_1): Handle polynomial
	numbers of units.
	(vect_schedule_slp_instance): Likewise.

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

From-SVN: r256141