our representation, since they are usually special cases of already-existing
instructions.
This abstracts away methods that let a pass create and verify a NOP instruction,
without relying on a `NOP' enum to be in existence in the target's instruction
info descriptor.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@6319 91177308-0d34-0410-b5e6-96231b3b80d8