[hir] Use a stable identity for undefined value

InferReferenceEffects uses object identity to merge states, which breaks when we 
create a new object to model `undefined`. 

Two value objects representing `undefined` are not equal due to referential 
equality. 

Instead, let's use a singleton to represent `undefined` value.

compiler/packages/babel-plugin-react-forget/src/Inference/InferReferenceEffects.ts

@@ -12,6 +12,7 @@ import {
   BlockId,
   CallExpression,
   Effect,
+  GeneratedSource,
   HIRFunction,
   IdentifierId,
   InstructionKind,
@@ -39,6 +40,12 @@ import {
 } from "../HIR/visitors";
 import { assertExhaustive } from "../Utils/utils";
 
+const UndefinedValue: InstructionValue = {
+  kind: "Primitive",
+  loc: GeneratedSource,
+  value: undefined,
+};
+
 /**
  * For every usage of a value in the given function, infers the effect or action
  * taken at that reference. Each reference is inferred as exactly one of:
@@ -933,11 +940,7 @@ function inferBlock(
         continue;
       }
       case "DeclareLocal": {
-        const value: InstructionValue = {
-          kind: "Primitive",
-          loc: instrValue.loc,
-          value: undefined,
-        };
+        const value = UndefinedValue;
         state.initialize(
           value,
           // Catch params may be aliased to mutable values

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/for-loop-let-undefined-decl.expect.md

@@ -0,0 +1,46 @@
+
+## Input
+
+```javascript
+function useFoo() {
+  for (let i = 0; i <= 5; i++) {
+    let color;
+    if (isSelected) {
+      color = isCurrent ? "#FFCC22" : "#FF5050";
+    } else {
+      color = isCurrent ? "#CCFF03" : "#CCCCCC";
+    }
+    console.log(color);
+  }
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  params: [],
+  fn: useFoo,
+};
+
+```
+
+## Code
+
+```javascript
+function useFoo() {
+  for (let i = 0; i <= 5; i++) {
+    let color = undefined;
+    if (isSelected) {
+      color = isCurrent ? "#FFCC22" : "#FF5050";
+    } else {
+      color = isCurrent ? "#CCFF03" : "#CCCCCC";
+    }
+
+    console.log(color);
+  }
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  params: [],
+  fn: useFoo,
+};
+
+```
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/for-loop-let-undefined-decl.js

@@ -0,0 +1,16 @@
+function useFoo() {
+  for (let i = 0; i <= 5; i++) {
+    let color;
+    if (isSelected) {
+      color = isCurrent ? "#FFCC22" : "#FF5050";
+    } else {
+      color = isCurrent ? "#CCFF03" : "#CCCCCC";
+    }
+    console.log(color);
+  }
+}
+
+export const FIXTURE_ENTRYPOINT = {
+  params: [],
+  fn: useFoo,
+};

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/loop-unused-let.expect.md

@@ -0,0 +1,21 @@
+
+## Input
+
+```javascript
+function useFoo() {
+  while (1) {
+    let foo;
+  }
+}
+
+```
+
+## Code
+
+```javascript
+function useFoo() {
+  while (1) {}
+}
+
+```
+      

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/loop-unused-let.js

@@ -0,0 +1,5 @@
+function useFoo() {
+  while (1) {
+    let foo;
+  }
+}

compiler/packages/sprout/src/SproutTodoFilter.ts

@@ -461,6 +461,7 @@ const skipFilter = new Set([
   "fbtparam-with-jsx-fragment-value",
   "fbt-preserve-jsxtext",
   "useContext-mutable-value",
+  "loop-unused-let",
 ]);
 
 export default skipFilter;