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

[Simon Pilgrim via llvm-commits]

https://github.com/RKSimon approved this pull request.

LGTM - good to see the helper being used more extensively. 

No objections to a similar cleanup for getConstantOperandAPInt - I'd add that fuzz tests often fall over because we've used getConstantOperandVal for types > i64 and we should have used getConstantOperandAPInt in the first place.

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