dillon1000/react

Improved check for intermediate values for merged scopes

We can't merge scopes if the intervening instructions produce values that are 
used later on. This PR improves the mechanism for detecting this case: first we 
build up a mapping of the last time (max instruction id) each identifier is 
used. Then when we're about to merge scopes we check if all the intervening 
lvalues are last used at or before the scope. If so that means it's safe to 
merge. 

I can't think of any edge cases that are problematic in the old behavior, but 
this version is more trivially correct and should allow us to extend to other 
types of instructions more easily.