Use tree_vector_builder::new_binary_operation for folding

Use tree_vector_builder::new_binary_operation for folding

This patch makes fold-const.c operate directly on the VECTOR_CST
encoding when folding an operation that has two VECTOR_CST inputs.

2017-12-07  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* tree-vector-builder.h
	(tree_vector_builder::new_binary_operation): Declare.
	* tree-vector-builder.c
	(tree_vector_builder::new_binary_operation): New function.
	* fold-const.c (fold_relational_const): Use it.
	(const_binop): Likewise.  Check that both input vectors have
	the same number of elements, thus excluding things like WIDEN_SUM.
	Check whether it is possible to operate directly on the encodings
	of stepped inputs.

From-SVN: r255477