poly_int: vectorizable_conversion

poly_int: vectorizable_conversion

This patch makes vectorizable_conversion cope with variable-length
vectors.  We already require the number of elements in one vector
to be a multiple of the number of elements in the other vector,
so the patch uses that to choose between widening and narrowing.

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

gcc/
	* tree-vect-stmts.c (vectorizable_conversion): Treat the number
	of units as polynomial.  Choose between WIDE and NARROW based
	on multiple_p.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>
From-SVN: r256139