[BOLT] removeAllSuccessors: handle multiple edges between basic blocks

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)