[SelectionDAG] Make SelectionDAG aware of the known bits in USUBO and SSUBO and SUBC.

Summary:
Depends on D30379

This improves the state of things for the sub class of operation.

Reviewers: jyknight, nemanjai, mkuper, spatel, RKSimon, zvi, bkramer

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D30436

llvm-svn: 297482