Handle bundled terminators in isBlockOnlyReachableByFallthrough.

Targets like SPARC and MIPS have delay slots and normally bundle the
delay slot instruction with the corresponding terminator.

Teach isBlockOnlyReachableByFallthrough to find any MBB operands on
bundled terminators so SPARC doesn't need to specialize this function.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199061 91177308-0d34-0410-b5e6-96231b3b80d8