dillon1000/react

Distinguish identifier id and ssa id

I noticed on @kassens's #771 that despite running LeaveSSA there are still cases 
where we still reassign to a unique identifier: functions that have reassignment 
but no phi nodes, such as: 

```javascript 

function foo() { 

let x$1 = 0; 

x$2 = x$1 + 1; 

} 

``` 

Here SSA form rewrote the second statement's LHS, but bc there's no phi node we 
can't recover what the original was supposed to be (`x = x + 1`). This was my 
oversight when suggesting the simpler LeaveSSA algorithm, it works for 
eliminating phis but not other reassignments. The only alternative to removing 
SSA form is to add assignment statements, which we obviously don't want to do 
since that generates bloat. 

This PR addresses the issue by adding an additional, optional property to 
`Identifier` called `preSsaId` that starts off null. When entering SSA we save 
the original id in this property and update id to a new SSA value. LeaveSSA does 
the inverse, setting id = preSsaId and nulling out the latter. This means that 
an identifier can always be uniquely identified by its `id` value at any point 
in the compiler, while it's trivial to correctly undo SSA form. 

```typescript 

type Identifier = { 

// Unique value for each original identifier 

id: IdentifierId; 

// The original, un-mangled variable name if this was a variable present in the 
source (null if it's generated) 

name: string | null; 

// When in SSA mode, this is set to the original, pre-SSA `id` value 

preSsaId: IdentifierId | null; 

} 

```