[LLE] Use the PredicatedScalarEvolution interface to query SCEVs for dependences

Summary:
LAA uses the PredicatedScalarEvolution interface, so it can produce
forward/backward dependences having SCEVs that are AddRecExprs only after being
transformed by PredicatedScalarEvolution.

Use PredicatedScalarEvolution to get the expected expressions.

Reviewers: anemet

Subscribers: llvm-commits, sanjoy

Differential Revision: http://reviews.llvm.org/D15382

llvm-svn: 255241