rs6000: Fold xxsel to vsel since they have same semantics

rs6000: Fold xxsel to vsel since they have same semantics

Fold xxsel to vsel like xxperm/vperm to avoid duplicate code.

gcc/ChangeLog:

2021-10-28  Xionghu Luo  <luoxhu@linux.ibm.com>

	PR target/94613
	* config/rs6000/altivec.md: Add vsx register constraints.
	* config/rs6000/vsx.md (vsx_xxsel<mode>): Delete.
	(vsx_xxsel<mode>2): Likewise.
	(vsx_xxsel<mode>3): Likewise.
	(vsx_xxsel<mode>4): Likewise.

gcc/testsuite/ChangeLog:

2021-10-28  Xionghu Luo  <luoxhu@linux.ibm.com>

	* gcc.target/powerpc/builtins-1.c: Adjust.