[compiler] Fix for consecutive DCE'd branches with phis

This is an optimized version of @asmjmp0's fix in https://github.com/facebook/react/pull/31940. When we merge consecutive blocks we need to take care to rewrite later phis whose operands will now be different blocks due to merging. Rather than iterate all the blocks on each merge as in #31940, we can do a single iteration over all the phis at the end to fix them up.

Note: this is a redo of #31959

compiler/packages/babel-plugin-react-compiler/src/HIR/MergeConsecutiveBlocks.ts

@@ -107,6 +107,17 @@ export function mergeConsecutiveBlocks(fn: HIRFunction): void {
     merged.merge(block.id, predecessorId);
     fn.body.blocks.delete(block.id);
   }
+  for (const [, block] of fn.body.blocks) {
+    for (const phi of block.phis) {
+      for (const [predecessorId, operand] of phi.operands) {
+        const mapped = merged.get(predecessorId);
+        if (mapped !== predecessorId) {
+          phi.operands.delete(predecessorId);
+          phi.operands.set(mapped, operand);
+        }
+      }
+    }
+  }
   markPredecessors(fn.body);
   for (const [, {terminal}] of fn.body.blocks) {
     if (terminalHasFallthrough(terminal)) {

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/repro-missing-phi-after-dce-merge-scopes.expect.md

@@ -0,0 +1,52 @@
+
+## Input
+
+```javascript
+function Component() {
+  let v3, v4, acc;
+  v3 = false;
+  v4 = v3;
+  acc = v3;
+  if (acc) {
+    acc = true;
+    v3 = acc;
+  }
+  if (acc) {
+    v3 = v4;
+  }
+  v4 = v3;
+  return [acc, v3, v4];
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [],
+};
+
+```
+
+## Code
+
+```javascript
+import { c as _c } from "react/compiler-runtime";
+function Component() {
+  const $ = _c(1);
+  let t0;
+  if ($[0] === Symbol.for("react.memo_cache_sentinel")) {
+    t0 = [false, false, false];
+    $[0] = t0;
+  } else {
+    t0 = $[0];
+  }
+  return t0;
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [],
+};
+
+```
+      
+### Eval output
+(kind: ok) [false,false,false]

compiler/packages/babel-plugin-react-compiler/src/__tests__/fixtures/compiler/repro-missing-phi-after-dce-merge-scopes.js

@@ -0,0 +1,20 @@
+function Component() {
+  let v3, v4, acc;
+  v3 = false;
+  v4 = v3;
+  acc = v3;
+  if (acc) {
+    acc = true;
+    v3 = acc;
+  }
+  if (acc) {
+    v3 = v4;
+  }
+  v4 = v3;
+  return [acc, v3, v4];
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  fn: Component,
+  params: [],
+};