dillon1000/react

[hir] Add support for handling phis in InferReferenceEffects

This just piggybacks on the infrastructure for handling Env.#variables. 

In the future, a better approach would be to simplify the environment creation 
and merging by leveraging the SSA property of the new IR -- 1) We don't need to 
track IdentifierId per environment as they are all    unique 2) Rather than 
tracking values, we can just track Identifiers because    Identifiers can never 
be reassigned.