Refine the definition of convergent to only disallow the addition of new control...

Refine the definition of convergent to only disallow the addition of new control dependencies.

This covers the common case of operations that cannot be sunk.
Operations that cannot be hoisted should already be handled properly via
the safe-to-speculate rules and mechanisms.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249865 91177308-0d34-0410-b5e6-96231b3b80d8