[IR] Add {is,set,setNot}Convergent() functions to CallSite, CallInstr, and InvokeInstr.

Summary:
(CallSite already has isConvergent() and setConvergent().)

No functional changes.

Reviewers: reames

Subscribers: llvm-commits, jingyue, arsenm

Differential Revision: http://reviews.llvm.org/D17316

llvm-svn: 261112