[SCEV] Change the SCEV Predicates interfaces for conversion to AddRecExpr to...

[SCEV] Change the SCEV Predicates interfaces for conversion to AddRecExpr to return SCEVAddRecExpr* instead of SCEV*

Summary:
This changes the conversion functions from SCEV * to SCEVAddRecExpr from
ScalarEvolution and PredicatedScalarEvolution to return a SCEVAddRecExpr*
instead of a SCEV* (which removes the need of most clients to do a
dyn_cast right after calling these functions).

We also don't add new predicates if the transformation was not successful.

This is not entirely a NFC (as it can theoretically remove some predicates
from LAA when we have an unknown dependece), but I couldn't find an obvious
regression test for it.

Reviewers: sanjoy

Subscribers: sanjoy, mzolotukhin, llvm-commits

Differential Revision: http://reviews.llvm.org/D18368

llvm-svn: 264161