[SCEV] Use a SmallPtrSet as a temporary union predicate; NFC

[SCEV] Use a SmallPtrSet as a temporary union predicate; NFC

Summary:
Instead of creating and destroying SCEVUnionPredicate instances (which
internally creates and destroys a DenseMap), use temporary SmallPtrSet
instances of remember the set of predicates that will get reified into a
SCEVUnionPredicate.

Reviewers: silviu.baranga, sbaranga

Subscribers: sanjoy, mcrosier, llvm-commits, mzolotukhin

Differential Revision: https://reviews.llvm.org/D25000

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@282606 91177308-0d34-0410-b5e6-96231b3b80d8