poly_int: vectorizable_call

This patch makes vectorizable_call handle variable-length vectors.
The only substantial change is to use build_index_vector for
IFN_GOMP_SIMD_LANE; this makes no functional difference for
fixed-length vectors.

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_call): Treat the number of
	vectors as polynomial.  Use build_index_vector for
	IFN_GOMP_SIMD_LANE.

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

From-SVN: r256137