[MachineBlockPlacement] Let the target optimize the branches at the end.

After the layout of the basic blocks is set, the target may be able to get rid
of unconditional branches to fallthrough blocks that the generic code does not
catch. This happens any time TargetInstrInfo::AnalyzeBranch is not able to
analyze all the branches involved in the terminators sequence, while still
understanding a few of them.

In such situation, AnalyzeBranch can directly modify the branches if it has been
instructed to do so.

This patch takes advantage of that.

llvm-svn: 268328