dillon1000/react

Fix assignment expression with context variables

Fixes the one case discovered in the previous PR; for AssignmentExpression we 
correctly lowered the store instruction to a local/context, but then always used 
a `LoadLocal` to read the result back. 

The load instruction appears like it might be dangling - i think what was 
happening is that DCE cleaned up the unused LoadLocal whereas it leaves the 
LoadContext alone. But this works for now, we can always clean up the extra 
instruction later since this case isn't too common.