Dont count throw as an exit node (React semantics)

React will retry or abort components that throw (depending on a few conditions), 
so from React's perspective a `throw` statement is not a normal exit node. Thus 
the Rules of Hooks really have a caveat: the set of hooks that are called _in an 
execution that returns successfully_ must be consistent. Examples such as the 
following are therefore allowed: 

```javascript 

function Component(props) { 

if (props.cond) { 

throw new Error(...); 

} 

useHook(); 

} 

``` 

By modeling `throw` as an exit node, we rejected cases such as this. This diff 
changes to not model throws as exit nodes. #1584  changes this to make it an 
option, since some cases will want to consider throw as an exit node.

compiler/forget/src/HIR/Dominator.ts

@@ -113,10 +113,9 @@ class Dominator<T> {
    */
   get(id: T): T | null {
     const dominator = this.#nodes.get(id);
-    invariant(
-      dominator !== undefined,
-      `Called on invalid node identifier '${id}'`
-    );
+    if (dominator === undefined) {
+      return null;
+    }
     return dominator === id ? null : dominator;
   }
 
@@ -173,7 +172,7 @@ function computeReverseGraph(fn: HIRFunction): Graph<BlockId> {
       preds: new Set(eachTerminalSuccessor(block.terminal)),
       succs: new Set(block.preds),
     };
-    if (block.terminal.kind === "return" || block.terminal.kind === "throw") {
+    if (block.terminal.kind === "return") {
       node.preds.add(exitId);
       exit.succs.add(id);
     }