[PredicateInfo] Stable sort ValueDFS to remove non-deterministic ordering

Summary: This fixes failure in Transforms/Util/PredicateInfo/testandor.ll uncovered by D39245.

Reviewers: dberlin

Reviewed By: dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D39630

llvm-svn: 318165