dillon1000/react

HIRVisitor - change labels to block ids

HIRTreeVisitor previously passed a string label (for certain blocks). This 
changes to pass the raw BlockId, and have codegen convert that to a string. I'm 
not sure if we'll need this but it would be helpful for eg visiting the IR and 
emitting a new IR, while mapping block ids forward. Even if we don't need that 
it makes sense for Codegen to decide how to convert a block id into a label 
(which has to obey the rules of an identifier, not the visitor's concern).