[IfConversion] Fix duplicate successor error after if-converter.

[ivafanas]

Bug is detected for Simple case when FalseBB is a successor of both blocks BB and TrueBB.

It is a mistake to add FalseBB as successor of BB after copy-and-merging of TrueBB into BB. MachineVerifier reports an error on duplicate successors and predecessors in this case.

Approximate CFG to trigger the bug:

    BB
   |  \       /
   |   \     /
   |    TrueBB
   |   /     \
   |  /       \
  FalseBB      ...

It is important that branch in TrueBB is not analyzable.

Bug is triggered on the custom out-of-tree backend. Unfortunately, I'm not familiar good enough with other backends to write a good .mir test for this with not analyzable branch. Sorry for that. But fix seems too trivial.

[ivafanas]

@ostannard @ahatanaka @MatzeB

Could you please review the PR?
Seems like IfConversion.cpp does not have direct owners, and you are the most involved available developers who changed the file.

I do not have write access.
If patch is ok, please, feel free to merge it :)

[ivafanas]

Please, any feedback?

Even PR rejection is much better than no feedback.

[ostannard]

I'd prefer to see a test for this before committing it. I'm not very familiar with the if-conversion pass, so it's not clear whether this is the correct fix, or if we should be avoiding doing this optimisation for non-analyzable branches.