[hir] Don't redefine context refs in EnterSSA

Context references are special since reassignments need to update the original 
reference, not define a new place.

compiler/forget/src/SSA/EnterSSA.ts

@@ -36,6 +36,7 @@ class SSABuilder {
   #blocks: Map<BlockId, BasicBlock>;
   #env: Environment;
   #unknown: Set<Identifier> = new Set();
+  #context: Set<Identifier> = new Set();
 
   constructor(env: Environment, blocks: Map<BlockId, BasicBlock>) {
     this.#blocks = blocks;
@@ -67,6 +68,12 @@ class SSABuilder {
     };
   }
 
+  defineContext(oldPlace: Place): Place {
+    const newPlace = this.definePlace(oldPlace);
+    this.#context.add(oldPlace.identifier);
+    return newPlace;
+  }
+
   definePlace(oldPlace: Place): Place {
     const oldId = oldPlace.identifier;
     if (this.#unknown.has(oldId)) {
@@ -78,6 +85,11 @@ class SSABuilder {
       );
     }
 
+    // Do not redefine context references.
+    if (this.#context.has(oldId)) {
+      return this.getPlace(oldPlace);
+    }
+
     const newId = this.makeId(oldId);
     this.state().defs.set(oldId, newId);
     return {
@@ -206,7 +218,7 @@ export default function enterSSA(func: HIRFunction): void {
     builder.startBlock(block);
 
     if (func.body.entry === blockId) {
-      func.context = func.context.map((p) => builder.definePlace(p));
+      func.context = func.context.map((p) => builder.defineContext(p));
       func.params = func.params.map((p) => builder.definePlace(p));
     }
 

compiler/forget/src/__tests__/fixtures/hir/capturing-func-simple-alias.expect.md

@@ -18,29 +18,20 @@ function component(a) {
 
 ```javascript
 function component(a) {
-  const $ = React.unstable_useMemoCache(4);
+  const $ = React.unstable_useMemoCache(2);
   const c_0 = $[0] !== a;
-  let t0;
-  if (c_0) {
-    t0 = { a: a };
-    $[0] = a;
-    $[1] = t0;
-  } else {
-    t0 = $[1];
-  }
-  const x = t0;
-  const c_2 = $[2] !== x;
   let y;
-  if (c_2) {
+  if (c_0) {
+    const x = { a: a };
     y = {};
     (function () {
       y = x;
     })();
     mutate(y);
-    $[2] = x;
-    $[3] = y;
+    $[0] = a;
+    $[1] = y;
   } else {
-    y = $[3];
+    y = $[1];
   }
   return y;
 }