dillon1000/react

[validation] Patch false positives for hook calls after loops

--- 

We were throwing `InvalidReact` errors on valid inputs. 

```js 

// Input 

function Foo() { 

log("block0"); 

for (const _ of foo) { 

log("loop"); 

} 

useBar(); 

} 

// IR 

bb0: 

// log("block0"); 

ForOf init=bb2 loop=bb3 fallthrough=bb1 

bb2: 

// init 

Branch: then:bb3 else:bb1 

bb3: 

// log("loop") 

Goto(Continue) bb2 

bb1: 

// useBar(); 

Return 

``` 

We correctly compute post dominators here. 

```js 

// In validateUnconditionalHooks 

console.log(dominators.debug()); 

/* Output: 

(read x => y as x is the post-dominator for y (all paths from x to the exit must 
go through y)) 

"bb4" => "bb4", 

"bb1" => "bb4", 

"bb2" => "bb1", 

"bb3" => "bb2", 

"bb0" => "bb2", 

*/ 

``` 

However, `findBlocksWithBackEdges` prevented us from adding `bb1` to the 
`unconditionalBlocks` set as `bb2` (its post dominator) has a back edge. I'm not 
sure what the `findBlocksWithBackEdges` was doing previously, so I replaced it 
with an invariant asserting that the loop terminates.