LabelTerminal scaffolding

Adds a new `LabelTerminal` which will be used to represent LabeledStatements 
that contain a statement other than a loop. What we do for these cases is 
basically break the containing block in two, with a goto after the inner 
statement to the fallthrough. This allows us to model the label, and any `break` 
to it, in the HIR. However this fails in codegen because we can't find the 
fallthrough branch — we need a high level terminal that knows about this 
structure. 

Hence LabelTerminal. Now, instead of just a continuation block and a goto, we 
have a structured terminal. The LabelTerminal expresses the block for the 
labeled statement and the continuation, and we can use this to put it back 
together when constructing a ReactiveFunction. Note that this PR is just the 
scaffolding for LabelTerminal, the next PR is the interesting bits.

compiler/forget/src/HIR/HIR.ts

@@ -269,7 +269,8 @@ export type Terminal =
   | WhileTerminal
   | LogicalTerminal
   | TernaryTerminal
-  | OptionalCallTerminal;
+  | OptionalCallTerminal
+  | LabelTerminal;
 
 /**
  * Terminal nodes allowed for a value block
@@ -379,6 +380,14 @@ export type TernaryTerminal = {
   loc: SourceLocation;
 };
 
+export type LabelTerminal = {
+  kind: "label";
+  block: BlockId;
+  fallthrough: BlockId | null;
+  id: InstructionId;
+  loc: SourceLocation;
+};
+
 export type OptionalCallTerminal = {
   kind: "optional-call";
   // Whether the call itself is optional. If false, this means that the callee itself

compiler/forget/src/HIR/HIRBuilder.ts

@@ -667,6 +667,10 @@ export function reversePostorderBlocks(func: HIR): void {
         visit(terminal.init);
         break;
       }
+      case "label": {
+        visit(terminal.block);
+        break;
+      }
       case "unsupported": {
         break;
       }

compiler/forget/src/HIR/PrintHIR.ts

@@ -79,6 +79,7 @@ export function printMixedHIR(
     return printInstruction(value);
   }
   switch (value.kind) {
+    case "label":
     case "optional-call":
     case "branch":
     case "if":
@@ -213,6 +214,12 @@ export function printTerminal(terminal: Terminal): Array<string> | string {
       value = `[${terminal.id}] ForOf init=bb${terminal.init} loop=bb${terminal.loop} fallthrough=bb${terminal.fallthrough}`;
       break;
     }
+    case "label": {
+      value = `[${terminal.id}] Label block=bb${terminal.block} fallthrough=${
+        terminal.fallthrough ? `bb${terminal.fallthrough}` : ""
+      }`;
+      break;
+    }
     case "unsupported": {
       value = `Unsupported`;
       break;

compiler/forget/src/HIR/visitors.ts

@@ -659,6 +659,18 @@ export function mapTerminalSuccessors(
         id: makeInstructionId(0),
       };
     }
+    case "label": {
+      const block = fn(terminal.block);
+      const fallthrough =
+        terminal.fallthrough !== null ? fn(terminal.fallthrough) : null;
+      return {
+        kind: "label",
+        block,
+        fallthrough,
+        id: makeInstructionId(0),
+        loc: terminal.loc,
+      };
+    }
     case "unsupported": {
       return terminal;
     }
@@ -726,6 +738,10 @@ export function* eachTerminalSuccessor(terminal: Terminal): Iterable<BlockId> {
       yield terminal.init;
       break;
     }
+    case "label": {
+      yield terminal.block;
+      break;
+    }
     case "unsupported":
       break;
     default: {
@@ -767,6 +783,7 @@ export function mapTerminalOperands(
       }
       break;
     }
+    case "label":
     case "optional-call":
     case "ternary":
     case "logical":
@@ -815,6 +832,7 @@ export function* eachTerminalOperand(terminal: Terminal): Iterable<Place> {
       }
       break;
     }
+    case "label":
     case "optional-call":
     case "ternary":
     case "logical":

compiler/forget/src/ReactiveScopes/BuildReactiveFunction.ts

@@ -29,6 +29,7 @@ import {
   ReactiveValue,
   Terminal,
 } from "../HIR/HIR";
+import todo from "../Utils/todo";
 import { assertExhaustive } from "../Utils/utils";
 
 /**
@@ -529,6 +530,9 @@ class Driver {
 
         break;
       }
+      case "label": {
+        todo("Support label terminals");
+      }
       case "optional-call":
       case "ternary":
       case "logical": {

compiler/forget/src/Utils/VisualizeHIRMermaid.ts

@@ -131,6 +131,10 @@ function printTerminalLabel(terminal: Terminal): string {
     case "for-of":
       buffer.push("ForOf");
       break;
+    case "label": {
+      buffer.push("Label");
+      break;
+    }
     case "unsupported": {
       break;
     }
@@ -224,6 +228,15 @@ function printTerminalArrows(blockId: BlockId, terminal: Terminal): string {
       buffer.push(printJumpArrow(blockId, terminal.loop, "loop"));
       break;
     }
+    case "label": {
+      buffer.push(printJumpArrow(blockId, terminal.block, "block"));
+      if (terminal.fallthrough != null) {
+        buffer.push(
+          printJumpArrow(blockId, terminal.fallthrough, "fallthrough")
+        );
+      }
+      break;
+    }
     case "throw":
     case "return":
     case "unsupported": {