poly_int: vectorizable_simd_clone_call

This patch makes vectorizable_simd_clone_call cope with variable-length
vectors.  For now we don't support SIMD clones for variable-length
vectors; this will be post GCC 8 material.

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 (simd_clone_subparts): New function.
	(vectorizable_simd_clone_call): Use it instead of TYPE_VECTOR_SUBPARTS.

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

From-SVN: r256138