[compiler] Flatten scopes based on fallthrough, not scope range

Once we create scopes, we should prefer to use the block structure to identify active scope ranges rather than the scope range. They _should_ always be in sync, but ultimately the block structure determine the active range (ie the id of the 'scope' terminal and the terminal's fallthrough block).

ghstack-source-id: 730b6d1
Pull Request resolved: #30398

Author: josephsavona

compiler/packages/babel-plugin-react-compiler/src/ReactiveScopes/FlattenScopesWithHooksOrUseHIR.ts

@@ -11,7 +11,6 @@ import {
   HIRFunction,
   LabelTerminal,
   PrunedScopeTerminal,
-  ReactiveScope,
   getHookKind,
   isUseOperator,
 } from '../HIR';
@@ -39,12 +38,11 @@ import {retainWhere} from '../Utils/utils';
  * to ensure the hook call does not inadvertently become conditional.
  */
 export function flattenScopesWithHooksOrUseHIR(fn: HIRFunction): void {
-  const activeScopes: Array<{block: BlockId; scope: ReactiveScope}> = [];
+  const activeScopes: Array<{block: BlockId; fallthrough: BlockId}> = [];
   const prune: Array<BlockId> = [];
 
   for (const [, block] of fn.body.blocks) {
-    const firstId = block.instructions[0]?.id ?? block.terminal.id;
-    retainWhere(activeScopes, current => current.scope.range.end > firstId);
+    retainWhere(activeScopes, current => current.fallthrough !== block.id);
 
     for (const instr of block.instructions) {
       const {value} = instr;
@@ -66,7 +64,7 @@ export function flattenScopesWithHooksOrUseHIR(fn: HIRFunction): void {
     if (block.terminal.kind === 'scope') {
       activeScopes.push({
         block: block.id,
-        scope: block.terminal.scope,
+        fallthrough: block.terminal.fallthrough,
       });
     }
   }