Pass vec_perm_indices by reference

This patch makes functions take vec_perm_indices by reference rather
than value, since a later patch will turn vec_perm_indices into a class
that would be more expensive to copy.

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

gcc/
	* optabs-query.h (can_vec_perm_p): Take a const vec_perm_indices *.
	* optabs-query.c (can_vec_perm_p): Likewise.
	* fold-const.c (fold_vec_perm): Take a const vec_perm_indices &
	instead of vec_perm_indices.
	* tree-vectorizer.h (vect_gen_perm_mask_any): Likewise,
	(vect_gen_perm_mask_checked): Likewise,
	* tree-vect-stmts.c (vect_gen_perm_mask_any): Likewise,
	(vect_gen_perm_mask_checked): Likewise,

From-SVN: r256090