[SCEV] Factor out isKnownViaInduction. NFC.

This just extracts the isKnownViaInduction from isKnownPredicate.

Reviewers: sanjoy, mkazantsev, reames
Reviewed By: mkazantsev
Subscribers: llvm-commits
Differential Revision: https://reviews.llvm.org/D44554

llvm-svn: 327824