Make BinaryFunction::fixBranches() more flexible and support CFG updates.

Summary:
The CFG represents "the ultimate source of truth". Transformations
on functions and blocks have to update the CFG and fixBranches() would
make sure the correct branch instructions are inserted at the end of
basic blocks (or removed when necessary).

We do require a conditional branch at the end of the basic block if
the block has 2 successors as CFG currently lacks the conditional
code support (it will probably stay that way). We only use this
branch instruction for its conditional code, the destination is
determined by CFG - first successor representing true/taken branch,
while the second successor - false/fall-through branch.

When we reverse the branch condition, the CFG is updated accordingly.

The previous version used to insert jumps after some terminating
instructions sometimes resulting in a larger code than needed. As a
result with the new version 1 extra function becomes overwritten for
HHVM binary.

With this diff we also convert conditional branches with one successor
(result of code from __builtin_unreachable()) into unconditional
jumps.

(cherry picked from FBD3802062)