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

This helper function shortens examples like
`cast<ConstantSDNode>(Node->getOperand(1))->getZExtValue();` to
`Node->getConstantOperandVal(1);`.

Implemented with:
`git grep -l
"cast<ConstantSDNode>\(.*->getOperand\(.*\)\)->getZExtValue\(\)" | xargs
sed -E -i

's/cast<ConstantSDNode>\((.*)->getOperand\((.*)\)\)->getZExtValue\(\)/\1->getConstantOperandVal(\2)/`
and `git grep -l
"cast<ConstantSDNode>\(.*\.getOperand\(.*\)\)->getZExtValue\(\)" | xargs
sed -E -i

's/cast<ConstantSDNode>\((.*)\.getOperand\((.*)\)\)->getZExtValue\(\)/\1.getConstantOperandVal(\2)/'`.
With a couple of simple manual fixes needed. Result then processed by
`git clang-format`.