poly_int: vect_get_constant_vectors

For now, vect_get_constant_vectors can only cope with constant-length
vectors, although a patch after the main SVE submission relaxes this.
This patch adds an appropriate guard for variable-length vectors.
The TYPE_VECTOR_SUBPARTS use in vect_get_constant_vectors will then
have a to_constant call when TYPE_VECTOR_SUBPARTS becomes a poly_int.

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_get_and_check_slp_defs): Reject
	constant and extern definitions for variable-length vectors.
	(vect_get_constant_vectors): Note that the number of units
	is known to be constant.

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

From-SVN: r256140