[SCEV] Don't add predicates already implied by UnionPredicate. (#93397)

Update SCEVUnionPredicate::add to only add predicates from another union
predicate, if they aren't alread implied by the union predicate we add
them to.

Note that there exists logic elsewhere to avoid adding predicates if
they are already implied, but this logic misses cases when only some
predicates of a union predicate are implied by the current set of
predicates.

PR: https://github.com/llvm/llvm-project/pull/93397