Use vec_concat for building up vector pair vectors.

Use vec_concat for building up vector pair vectors.

2023-11-16  Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/rs6000.cc (rs6000_expand_vector_pair_init): Rename
	gen_vpair_assemble_<mode> to gen_vpair_concat_<mode>.
	* config/rs6000/vector-pair.md (UNSPEC_VPAIR_ASSEMBLE): Delete
	(UNSPEC_VPAIR_SPLAT): Likewise.
	(vpair_concat_<mode>): Rename from vpair_assemble_<mode>.  Re-implement
	it, using vec_concat.
	(vpair_splat_<mode>): Likewise.