dillon1000/react

Throw CompilerError (todo) for unused conditional/logical

If a logical or conditional expression is unused, then a phi node isn't created 
for the identifier it assigns to. Then when we leave SSA form the two branches 
will assign to separate values, and we aren't sure which identifier to use as 
the lvalue of the resulting ReactiveInstruction (remember that 
logicals/conditionals decompose into control flow in HIR, but are a single 
compound instruction in ReactiveFunction). If the two sides don't assign to the 
same location, it could be because of a bug in the compiler or because the value 
wasn't used. Ideally we'd represent this explicitly, but for now i'm just making 
this a TODO since most logicals/conditionals should have their value used.