dillon1000/react

Run LeaveSSA prior to analyzing reactive scopes

Reorders LeaveSSA so that it runs before we begin evaluating reactive scopes. 
Note that reactive scopes must span the full construction of each variable — for 
variables with a phi, this must span the declaration and all assignments of the 
phi operands. And that's exactly what the new LeaveSSA does! LeaveSSA removes 
phi nodes and ensure that all versions of a variable which flow into a phi have 
been assigned a single canonical identifier (with an appropriate mutable range). 

This PR includes this and some related changes: 

* Reorders the pass 

* Changes hir-test to print the final HIR, eg just prior to codegen 

* Teaches LeaveSSA to update the mutable range of the canonical identifiers it 
assigns, based on the min/max of the variables assigned.