Make more use of gimple-fold.h in tree-vect-loop.c

This patch makes the vectoriser use the gimple-fold.h routines
in more cases, instead of vect_init_vector.  Later patches want
to use the same interface to handle variable-length vectors.

2017-09-14  Richard Sandiford  <richard.sandiford@linaro.org>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

gcc/
	* tree-vect-loop.c (vectorizable_induction): Use gimple_build instead
	of vect_init_vector.

Co-Authored-By: Alan Hayward <alan.hayward@arm.com>
Co-Authored-By: David Sherwood <david.sherwood@arm.com>

From-SVN: r252763