Add gimple_build_vector* helpers

Add gimple_build_vector* helpers

This patch adds gimple-fold.h equivalents of build_vector and
build_vector_from_val.  Like the other gimple-fold.h routines
they always return a valid gimple value and add any new
statements to a given gimple_seq.  In combination with later
patches this reduces the number of force_gimple_operands.

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

gcc/
	* gimple-fold.h (gimple_build_vector_from_val): Declare, and provide
	an inline wrapper that provides a location.
	(gimple_build_vector): Likewise.
	* gimple-fold.c (gimple_build_vector_from_val): New function.
	(gimple_build_vector): Likewise.
	* tree-vect-loop.c (get_initial_def_for_reduction): Use the new
	functions to build the initial value.  Always return a gimple value.
	(get_initial_defs_for_reduction): Likewise.  Only compute
	neutral_vec once.
	(vect_create_epilog_for_reduction): Don't call force_gimple_operand or
	vect_init_vector on the results from get_initial_def(s)_for_reduction.
	(vectorizable_induction): Use gimple_build_vector rather than
	vect_init_vector.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@252762 138bc75d-0d04-0410-961f-82ee72b054a4