[RFC][SelectionDAG] Add and use SDNode::getAsZExtVal() helper (#76710)

This follows on from #76708, allowing
`cast<ConstantSDNode>(N)->getZExtValue()` to be replaced with just
`N->getAsZextVal();`
    
Introduced via `git grep -l "cast<ConstantSDNode>\(.*\).*getZExtValue" |
xargs sed -E -i
's/cast<ConstantSDNode>\((.*)\)->getZExtValue/\1->getAsZExtVal/'` and
then using `git clang-format` on the result.