[hir] implement assignment expressions

Implements assignment expressions with operators other than `=` (such as `+=`) 
by lowering to an assignment. 

I think this isn't fully correct for something like `a.b.c += 1`, but it seems 
like there's more gaps in object accesses.

compiler/forget/src/HIR/BuildHIR.ts

@@ -901,13 +901,50 @@ function lowerExpression(
     case "AssignmentExpression": {
       const expr = exprPath as NodePath<t.AssignmentExpression>;
       const left = lowerLVal(builder, expr.get("left"));
-      const right = lowerExpression(builder, expr.get("right"));
       const operator = expr.node.operator;
-      todoInvariant(operator === "=", "todo: support non-simple assignment");
+
+      if (operator === "=") {
+        const right = lowerExpression(builder, expr.get("right"));
+        builder.push({
+          id: 0,
+          lvalue: { place: left, kind: InstructionKind.Reassign },
+          value: right,
+          loc: exprLoc,
+        });
+        return left;
+      }
+
+      const operators: { [key: string]: t.BinaryExpression["operator"] } = {
+        "+=": "+",
+        "-=": "-",
+        "/=": "/",
+        "%=": "%",
+        "*=": "*",
+        "**=": "**",
+        "&=": "&",
+        "|=": "|",
+        ">>=": ">>",
+        ">>>=": ">>>",
+        "<<=": "<<",
+        "^=": "^",
+      };
+      const binaryOperator = operators[operator];
+      invariant(
+        binaryOperator != null,
+        `Unhandled assignment operator '${operator}'`
+      );
+
+      const right = lowerExpressionToPlace(builder, expr.get("right"));
       builder.push({
         id: 0,
         lvalue: { place: left, kind: InstructionKind.Reassign },
-        value: right,
+        value: {
+          kind: "BinaryExpression",
+          operator: binaryOperator,
+          left,
+          right,
+          loc: exprLoc,
+        },
         loc: exprLoc,
       });
       return left;

compiler/forget/src/__tests__/fixtures/hir/assignment-variations.expect.md

@@ -0,0 +1,107 @@
+
+## Input
+
+```javascript
+function f() {
+  let x = 1;
+  x = x + 1;
+  x += 1;
+  x >>>= 1;
+}
+
+function g(a) {
+  a.b.c = a.b.c + 1;
+  a.b.c *= 2;
+}
+
+```
+
+## HIR
+
+```
+bb0:
+  [1] Let mutate x$5 = 1
+  [2] Const mutate $6 = 1
+  [3] Reassign mutate x$7 = Binary read x$5 + read $6
+  [4] Const mutate $8 = 1
+  [5] Reassign mutate x$9 = Binary read x$7 + read $8
+  [6] Const mutate $10 = 1
+  [7] Reassign mutate x$11 = Binary read x$9 >>> read $10
+  Return
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Let mutate x$5 = 1
+      [2] Const mutate $6 = 1
+      [3] Reassign mutate x$7 = Binary read x$5 + read $6
+      [4] Const mutate $8 = 1
+      [5] Reassign mutate x$9 = Binary read x$7 + read $8
+      [6] Const mutate $10 = 1
+      [7] Reassign mutate x$11 = Binary read x$9 >>> read $10
+    "]
+    bb0_instrs --> bb0_terminal(["Return"])
+  end
+
+  %% Jumps
+  %% empty
+```
+
+## Code
+
+```javascript
+function f$0() {
+  let x$5 = 1;
+  x$7 = x$5 + 1;
+  x$9 = x$7 + 1;
+  x$11 = x$9 >>> 1;
+  return;
+}
+
+```
+## HIR
+
+```
+bb0:
+  [1] Const mutate $5 = 1
+  [2] Reassign mutate a$4.b.c[0:4] = Binary read a$4.b.c + read $5
+  [3] Const mutate $6 = 2
+  [4] Reassign mutate a$4.b.c[0:4] = Binary read a$4.b.c * read $6
+  Return
+```
+
+### CFG
+
+```mermaid
+flowchart TB
+  %% Basic Blocks
+  subgraph bb0
+    bb0_instrs["
+      [1] Const mutate $5 = 1
+      [2] Reassign mutate a$4.b.c[0:4] = Binary read a$4.b.c + read $5
+      [3] Const mutate $6 = 2
+      [4] Reassign mutate a$4.b.c[0:4] = Binary read a$4.b.c * read $6
+    "]
+    bb0_instrs --> bb0_terminal(["Return"])
+  end
+
+  %% Jumps
+  %% empty
+```
+
+## Code
+
+```javascript
+function g$0(a$4) {
+  a$4 = a$4.b.c + 1;
+  a$4 = a$4.b.c * 2;
+  return;
+}
+
+```
+      

compiler/forget/src/__tests__/fixtures/hir/assignment-variations.js

@@ -0,0 +1,11 @@
+function f() {
+  let x = 1;
+  x = x + 1;
+  x += 1;
+  x >>>= 1;
+}
+
+function g(a) {
+  a.b.c = a.b.c + 1;
+  a.b.c *= 2;
+}