[CodeGen] fix broken successor probability in MBB dump

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@343297 91177308-0d34-0410-b5e6-96231b3b80d8