we consider arg->use_back() is not store inst as unsolved

Signed-off-by: xiehuc <xiehuc@gmail.com>
rename DDGNode to avoid ugly static_cast<DDGNode*>

Signed-off-by: xiehuc <xiehuc@gmail.com>
add a basic to_expr functionality

Signed-off-by: xiehuc <xiehuc@gmail.com>