[llvm] [SelectionDAG][NFC] Replace C-style casting with static_cast (PR #74060)

[Paul Kirth via llvm-commits]

ilovepi wrote:

I think this is mostly, correct, but can you put this commit in #73283, and close/abandon this PR? I don't think this fix should be a separate PR.

https://github.com/llvm/llvm-project/pull/74060