Fix a bug in IfConversion.cpp.

The bug is introduced in r254377 which failed some tests on ARM, where a new
probability is assigned to a successor but the provided BB may not be a
successor.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@254463 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed