Fix xxeval predicates (PR 99921).

I noticed that the xxeval built-in function used the altivec_register_operand
predicate.  Since it takes vsx registers, this might force the register
allocate to issue a move when it could use a traditional floating point
register.  This patch fixes that.

2021-08-13  Michael Meissner  <meissner@linux.ibm.com>

gcc/
	PR target/99921
	* config/rs6000/altivec.md (xxeval): Use register_predicate
	instead of altivec_register_predicate.