[DAG] Add knownbits/signbits handling for ISD::AVG* nodes

[RKSimon]

https://reviews.llvm.org/D106237 adds the ISD::AVGFLOORS/AVGFLOORU/AVGCEILS/AVGCEILU nodes which we will require value tracking handling for in SelectionDAG.

CC @davemgreen

[RKSimon]

ISD::AVGCEILU handling - https://reviews.llvm.org/D119629

[snikitav]

I can take this as well