This patch makes tree-vect-generic.c cope with variable-length vectors.
Decomposition is only supported for constant-length vectors, since we
should never generate unsupported variable-length operations.

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

gcc/
	* tree-vect-generic.c (nunits_for_known_piecewise_op): New function.
	(expand_vector_piecewise): Use it instead of TYPE_VECTOR_SUBPARTS.
	(expand_vector_addition, add_rshift, expand_vector_divmod): Likewise.
	(expand_vector_condition, vector_element): Likewise.
	(subparts_gt): New function.
	(get_compute_type): Use subparts_gt.
	(count_type_subparts): Delete.
	(expand_vector_operations_1): Use subparts_gt instead of
	count_type_subparts.

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

From-SVN: r256143