Make more use of rtx_vector_builder

Make more use of rtx_vector_builder

This patch makes various bits of CONST_VECTOR-building code use
rtx_vector_builder, operating directly on a specific encoding.

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

gcc/
	* expr.c: Include rtx-vector-builder.h.
	(const_vector_mask_from_tree): Use rtx_vector_builder and operate
	directly on the tree encoding.
	(const_vector_from_tree): Likewise.
	* optabs.c: Include rtx-vector-builder.h.
	(expand_vec_perm_var): Use rtx_vector_builder and create a repeating
	sequence of "u" values.
	* vec-perm-indices.c: Include rtx-vector-builder.h.
	(vec_perm_indices_to_rtx): Use rtx_vector_builder and operate
	directly on the vec_perm_indices encoding.

From-SVN: r256103