Fix bootstrap failure on SPARC with -O3 -mvis3

Fix bootstrap failure on SPARC with -O3 -mvis3

This replaces the use of FAIL in the new vec_cmp[u] expanders by that of a
predicate for the operator, which is (apparently) required for the optabs
machinery to properly compute the set of supported vector comparisons.

gcc/
	PR target/118096
	* config/sparc/predicates.md (vec_cmp_operator): New predicate.
	(vec_cmpu_operator): Likewise.
	* config/sparc/sparc.md (vec_cmp<FPCMP:mode><P:mode>): Use the
	vec_cmp_operator predicate instead of FAILing the expansion.
	(vec_cmpu<FPCMP:mode><P:mode>): Likewise for vec_cmpu_operator.