Use vec_perm_builder::series_p in shift_amt_for_vec_perm_mask

This patch makes shift_amt_for_vec_perm_mask use series_p to check
for the simple case of a natural linear series before falling back
to testing each element individually.  The series_p test works with
variable-length vectors but testing every individual element doesn't.

2018-01-02  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* optabs.c (shift_amt_for_vec_perm_mask): Try using series_p
	before testing each element individually.
	* tree-vect-generic.c (lower_vec_perm): Likewise.

From-SVN: r256099