Add a vec_perm_indices_to_tree helper function

This patch adds a function for creating a VECTOR_CST from a
vec_perm_indices, operating directly on the encoding.

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

gcc/
	* vec-perm-indices.h (vec_perm_indices_to_tree): Declare.
	* vec-perm-indices.c (vec_perm_indices_to_tree): New function.
	* tree-ssa-forwprop.c (simplify_vector_constructor): Use it.
	* tree-vect-slp.c (vect_transform_slp_perm_load): Likewise.
	* tree-vect-stmts.c (vectorizable_bswap): Likewise.
	(vect_gen_perm_mask_any): Likewise.

From-SVN: r256096