Update vector pair documentation; Add get vector built-in.

Update vector pair documentation; Add get vector built-in.

2023-08-04  Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/rs6000-builtins.def (__builtin_vpair_*_get_vector): Add
	built-ins.
	* config/rs6000/vector-pair.md (VPAIR_FP): Rename from VPAIR_FP_WRAPPER.
	(VPAIR_INT): Rename from VPAIR_INT_WRAPPER.
	(VPAIR_ALL): New int iterator.
	(vpair_get_vector_<vpair_type>): New insns.
	(vpair_*): Rename VPAIR_FP_WRAPPER to VPAIR_FP.  Rename
	VPAIR_INT_WRAPPER to VPAIR_INT.
	* doc/extend.texi (PowerPC Built-in Functions): Move where vector pair
	built-ins are documented.  Document __builtin_vpair_*_get_vector.