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

[Wang Pengcheng via llvm-commits]

wangpc-pp 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.

No, I think we should commit this seperately as these two PRs do different things. This PR is for fixing code style, the other is for reducing code size. WDYT? :-)

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