[CodeGen] fix broken successor probability in MBB dump
When printing successor probabilities for a MBB, a human readable value is sometimes shown as 200.0%.
The human readable output is based on getProbabilityIterator, which returns 0xFFFFFFFF for getNumerator() and 0x80000000 for getDenominator() for unknown BranchProbability.
By using getSuccProbability as we do for the non-human readable part, we can avoid this problem.
Differential Revision: https://reviews.llvm.org/D52605