[llvm] [llvm][NFC] Use SDValue::getConstantOperandVal(i) where possible (PR #76708)

[Alex Bradbury via llvm-commits]

asb wrote:

If this change is uncontroversial and people are happy with how it's done, I'd intend to directly commit the equivalent cleanup that moves to using the `getConstantOperandAPInt` helper where possible (unless people specifically think it's worth pre-commit review).

I can't reproduce the bot-suggested missing clang-format change locally, so it must be a slight difference in versions of clang-format or the git-clang-format helper script.

https://github.com/llvm/llvm-project/pull/76708