[SelectionDAG] Add and use SDNode::getAsAPIntVal() helper (#77455)

This is the logical equivalent for #76710 for APInt and uses the same
naming scheme.

Converted existing users through:
`git grep -l "cast<ConstantSDNode>\(.*\).*getAPIntValueValue" | xargs
sed -E -i
's/cast<ConstantSDNode>\((.*)\)->getAPIntValue/\1->getAsAPIntVal/'`