[LoopUnswitch]Fix comparison for DomTree updates.

Summary:
In LoopUnswitch when replacing a branch Parent -> Succ with a conditional
branch Parent -> True & Parent->False, the DomTree updates should insert an edge for
each of True/False if True/False are different than Succ, and delete Parent->Succ edge
if both are different. The comparison with Succ appears to be incorect,
it's comparing with Parent instead.
There is no test failing either before or after this change, but it seems to me this is
the right way to do the update.

Reviewers: chandlerc, kuhar

Subscribers: sanjoy, jlebar, llvm-commits

Differential Revision: https://reviews.llvm.org/D48457

llvm-svn: 335369