[JumpThread] Do RAUW in case Cond folds to a constant in the CFG

Summary: [JumpThread] Do RAUW in case Cond folds to a constant in the CFG

Reviewers: sanjoy

Reviewed By: sanjoy

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D32407

llvm-svn: 301804