[compiler] Fix for consecutive DCE'd branches with phis (#33725)

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

---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/33725).
* #33726
* __->__ #33725

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: [],
+};