dillon1000/react

Codegen for scopes without dependencies

Completes a todo for the `if` condition of scopes without any inputs. In this 
case since there are no inputs we can check for changes, we check if the first 
_output_ cache slot is set to the sentinel. 

For this to work we need to ensure that all scopes have at least one output, 
which isn't currently the case. Dead code can produce output-less sentinels. So 
this PR also adds a pass to find scopes w/o any outputs and convert them to 
regular blocks. We could in theory also just delete them, but for now let's be 
more conservative. This is something we'd want to highlight as a diagnostic in 
an IDE, though existing dead code linters would almost certainly find this case 
too.