[llvm][NFC] Use SDValue::getConstantOperandAPInt(i) where possible

The helper function allows examples like
`cast<ConstantSDNode>(Op.getOperand(0))->getAPIntValue();` to be changed
to `Op.getConstantOperandAPInt(0);`.

See #76708 for further context. Although there are far fewer
opportunities for replacement, I used a similar git grep and sed combo
as before, given I already had it to hand:

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