[llvm] [MachineSink] Extend loop sinking capability (PR #117247)

[Jeffrey Byrnes via llvm-commits]

jrbyrnes wrote:

Bring in https://github.com/llvm/llvm-project/pull/121818 -- this provides an interface for checking if an MI is dead.

As a result, I've deleted MachineSinking::isDead . The implementation concerns of isDead still exist in DeadMachineInstructionElim, but should be handeled separately from this.

https://github.com/llvm/llvm-project/pull/117247