Fix a conversion from a forward iterator to a reverse iterator in
MachineBasicBlock::SplitCriticalEdge. Since this is an iterator rather than
an instr_iterator, the isBundled() check only passes if getFirstTerminator()
returned end() and the garbage memory happens to lean that way.

Multiple successors can be present without any terminator instructions in the
case of exception handling with a fallthrough.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175383 91177308-0d34-0410-b5e6-96231b3b80d8