[KnownBits][ValueTracking] Move the math for calculating known bits for...

[KnownBits][ValueTracking] Move the math for calculating known bits for add/sub into a static method in KnownBits object

I want to reuse this code in SimplifyDemandedBits handling of Add/Sub. This will make that easier.

Wonder if we should use it in SelectionDAG's computeKnownBits too.

Differential Revision: https://reviews.llvm.org/D36433

llvm-svn: 310378