[llvm] [MachineSink] Only add sink candidate if toBB is a successor of fromBB (PR #98540)

[via llvm-commits]

yozhu wrote:

In the our internal application where this problem results in an assertion failure in `MachineBasicBlock::replaceSuccessor()`, there are a lots of `DeferredFromBB` discovered and only one of them is not immediate predecessor of the `ToBB`.

https://github.com/llvm/llvm-project/pull/98540