Fix missing OpPhi after merge return. (#2248)

The function `UpdatePhiNodes` was being called inconsistently.  In one
case, the cfg had already been updated to include the new edge, and in
another place the cfg was not updated.  This caused the function to
miss flagging a block as needing new phi nodes.  I picked that the cfg
should not be updated before making the call.  I documented it, and
change the call sites to match.

Fixes #2207.