[llvm] [LivePhysRegs] Make use of `MBB.liveouts()` (semi-NFC) (PR #154728)

[Benjamin Maxwell via llvm-commits]

MacDue wrote:

That error seems to be from before https://github.com/llvm/llvm-project/pull/155084 was applied (as that patch removed the "<function-style-cast>").

https://github.com/llvm/llvm-project/pull/154728