poly_int: vectorizable_induction

This patch makes vectorizable_induction cope with variable-length
vectors.  For now we punt on SLP inductions, but patchees after
the main SVE submission add support for those too.

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

gcc/
	* tree-vect-loop.c (vectorizable_induction): Treat the number
	of units as polynomial.  Punt on SLP inductions.  Use an integer
	VEC_SERIES_EXPR for variable-length integer reductions.  Use a
	cast of such a series for variable-length floating-point
	reductions.

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

From-SVN: r256134