[MachineSink] Address post-commit review comments

The successors cache is now a local variable, making it more visible that it
is only valid for the MBB being processed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239807 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed