dillon1000/react

Prune declarations not used after merging

This is the optimization mentioned in #2113. When we merge scopes, often the 
declarations from the first scope become unnecessary, since those values are 
only consumed by the subsequent, now-merged scope. There's no point emitting 
those values as outputs of the merged scope since no one can consume them. 

Thanks to the logic in #2116 we now know the last place each identifier is used. 
We use that again here, to prune declarations that aren't used past the end of 
the merged scope. This is a pretty dramatic win on cache slots used.