dillon1000/react

LeaveSSA: handle phi as operand to subsequent phi

Fixes https://github.com/facebook/react-forget/pull/858#discussion_r1044626137. 
The case is 

```javascript 

function foo() { 

let x$1 = 1; 

let y = 2; 

if (y === 2) { 

x$2 = 3; 

} 

x$3 = phi(x$1, x$2) 

if (y === 3) { 

x4 = 5; 

} 

x$5 = phi(x$3, x$4); 

y = x$5; 

} 

``` 

What happens here is that there are two _sequential_ phis for `x`. Previously 
when we encountered the second phi we would find that there is no `let` 
declaration for the phi or any of its operands, and create a new one before the 
second `if`. That's incorrect, these should all merge into a single `x` 
declaration. We now look up the phi operands to see if they are part of a 
previous phi, and merge them correctly.