Move gimplify_buildN API local to only remaining user

Move gimplify_buildN API local to only remaining user

This moves the legacy gimplify_buildN API to tree-vect-generic.c,
its only user and elides the gimplification step, making it a wrapper
around gimple_build, adjusting tree_vec_extract for this.

I've noticed that vector CTOR expansion doesn't deal with unfolded
{} and thus this makes it more resilent.  I've also adjusted the
match.pd vector CTOR extraction code to make sure it doesn't
produce a CTOR when folding would make it a vector constant.

2021-04-15  Richard Biener  <rguenther@suse.de>

	* tree-cfg.h (gimplify_build1): Remove.
	(gimplify_build2): Likewise.
	(gimplify_build3): Likewise.
	* tree-cfg.c (gimplify_build1): Move to tree-vect-generic.c.
	(gimplify_build2): Likewise.
	(gimplify_build3): Likewise.
	* tree-vect-generic.c (gimplify_build1): Move from tree-cfg.c.
	Modernize.
	(gimplify_build2): Likewise.
	(gimplify_build3): Likewise.
	(tree_vec_extract): Use resimplify with following SSA edges.
	(expand_vector_parallel): Avoid passing NULL size/bitpos
	to tree_vec_extract.
	* expr.c (store_constructor): Deal with zero-element CTORs.
	* match.pd (bit_field_ref <vector CTOR>): Make sure to
	produce vector constants when possible.