Restore "[LLVM] move verification of convergence control to a class template""

The refactored template can now be used with MachineVerifier.

Resubmitted after fixing build errors:

- Shared libraries build failed with undefined references due to "extern
  template" declarations.
- Modules build failed due to a cycle dependence between llvm/ADT and llvm/IR.
  The Generic*Impl.h files should be in llvm/IR to prevent this.

Differential Revision: https://reviews.llvm.org/D156522

This restores commit 93a37067.
Originally reverted in 466bd998.