[PredicateInfo] Fold PredicateWithCondition into PredicateBase (NFC).

Each concrete instance of a predicate has a condition (also noted in the
original PredicateBase comment) and to me it seems like there is no
clear benefit of having both PredicateBase and PredicateWithCondition
and they can be folded together.

Reviewers: nikic, efriedma

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D84089