dillon1000/react

Fix for IIFE return values mutated later

Fixes for the previous PR. What was happening is that our inference was 
inferring the correct mutable ranges and reactive scopes, but the inlining 
process left the instructions from the IIFEs inside a separate block, with a 
'label' terminal preceding it. When we converted to ReactiveFunction this was 
preserved as a ReactiveLabelTerminal, which meant that the first instruction for 
the mutable range could be nested inside one LabelTerminal, while more would be 
in a subsequent LabelTerminal. But we close blocks based on the block scope! 
This meant that we'd have leftover instructions (in the second LabelTerminal) 
that got left out of the block. 

Furthermore, because inlining was happening after EnterSSA we weren't creating 
phis correctly. This PR fixes a bunch of these issues, and a subsequent PR 
handles the remaining cases: 

* We move DropManualMemo and InlineIIFEs before EnterSSA. This means we lose the 
ability to use type information, but we ensure that we create proper SSA ids and 
phis for any reassignments within the IIFE 

* We also update PruneUnusedLabels to not just remove the unused labels, but to 
actually remove LabelTerminals that don't need them.