Summary:
If `addUnknownControlFlow` in `BinaryFunction::postProcessIndirectBranches`
is invoked with a basic block that has multiple edges to the same successor,
it leads to an assertion in `BinaryBasicBlock::removePredecessor`.
For basic blocks with multiple edges to the same successor, the default
behavior of removePredecessor is to remove all occurrences of the
predecessor block in its predecessor list (Multiple=true).
Example:
```A -> B (two edges)
A->removeAllSuccessors()
for each successor of block A: // B twice
// this removes both occurrences of A in B's predecessors list
B->removePredecessor(A);
// this invocation triggers an assert as A is no longer in B's
// predecessor list
B->removePredecessor(A);
```
This issue is not fixed by NormalizeCFG as `removeAllSuccessor` is called
earlier (from `buildCFG` -> `postProcessIndirectBranches`).
Solve this issue by collecting the successors into a set (`SmallPtrSet`) first,
before invoking `SuccessorBB->removePredecessor(this)`.
GitHub issue: https://github.com/facebookincubator/BOLT/issues/187
(cherry picked from FBD30796979)
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