[block-freq] Teach branch probability how to return the edge weight in between...

[block-freq] Teach branch probability how to return the edge weight in between a BasicBlock and one of its successors.

IMHO At some point BasicBlock should be refactored along the lines of
MachineBasicBlock so that successors/weights are actually embedded within the
block. Now is not that time though.

llvm-svn: 197303