Don't add CFG edges for redundant conditional branches.

Don't add CFG edges for redundant conditional branches.

IR that hasn't been through SimplifyCFG can look like this:

  br i1 %b, label %r, label %r

Make sure we don't create duplicate Machine CFG edges in this case.

Fix the machine code verifier to accept conditional branches with a
single CFG edge.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162230 91177308-0d34-0410-b5e6-96231b3b80d8