[valueblocks] Scaffolding for branch terminal

Scaffolding for a new 'branch' terminal, which is the lower-level form of 'if' 
meant for use in value blocks. See design at 
https://fb.quip.com/b7N6AjkDcl2a#temp:C:bAO904b0cf06f154de4a7287af6d

compiler/forget/src/HIR/HIR.ts

@@ -214,6 +214,7 @@ export type Terminal =
   | ReturnTerminal
   | GotoTerminal
   | IfTerminal
+  | BranchTerminal
   | SwitchTerminal
   | ForTerminal
   | WhileTerminal
@@ -256,6 +257,14 @@ export type IfTerminal = {
   id: InstructionId;
 };
 
+export type BranchTerminal = {
+  kind: "branch";
+  test: Place;
+  consequent: BlockId;
+  alternate: BlockId;
+  id: InstructionId;
+};
+
 export type SwitchTerminal = {
   kind: "switch";
   test: Place;

compiler/forget/src/HIR/HIRBuilder.ts

@@ -553,6 +553,12 @@ export function reversePostorderBlocks(func: HIR): void {
         visit(consequent);
         break;
       }
+      case "branch": {
+        const { consequent, alternate } = terminal;
+        visit(alternate);
+        visit(consequent);
+        break;
+      }
       case "switch": {
         // can ignore fallthrough, if its reachable it will be reached through
         // a case

compiler/forget/src/HIR/PrintHIR.ts

@@ -75,6 +75,7 @@ export function printMixedHIR(
     return printInstruction(value);
   }
   switch (value.kind) {
+    case "branch":
     case "if":
     case "logical":
     case "return":
@@ -134,6 +135,12 @@ export function printTerminal(terminal: Terminal): Array<string> | string {
       }`;
       break;
     }
+    case "branch": {
+      value = `[${terminal.id}] Branch (${printPlace(terminal.test)}) then:bb${
+        terminal.consequent
+      } else:bb${terminal.alternate}`;
+      break;
+    }
     case "logical": {
       value = `[${terminal.id}] Logical ${terminal.operator} test:bb${terminal.test} fallthrough=bb${terminal.fallthrough}`;
       break;

compiler/forget/src/HIR/visitors.ts

@@ -253,6 +253,17 @@ export function mapTerminalSuccessors(
         id: makeInstructionId(0),
       };
     }
+    case "branch": {
+      const consequent = fn(terminal.consequent);
+      const alternate = fn(terminal.alternate);
+      return {
+        kind: "branch",
+        test: terminal.test,
+        consequent,
+        alternate,
+        id: makeInstructionId(0),
+      };
+    }
     case "switch": {
       const cases = terminal.cases.map((case_) => {
         const target = fn(case_.block);
@@ -350,6 +361,11 @@ export function* eachTerminalSuccessor(terminal: Terminal): Iterable<BlockId> {
       yield terminal.alternate;
       break;
     }
+    case "branch": {
+      yield terminal.consequent;
+      yield terminal.alternate;
+      break;
+    }
     case "switch": {
       for (const case_ of terminal.cases) {
         yield case_.block;
@@ -394,6 +410,10 @@ export function mapTerminalOperands(
       terminal.test = fn(terminal.test);
       break;
     }
+    case "branch": {
+      terminal.test = fn(terminal.test);
+      break;
+    }
     case "switch": {
       terminal.test = fn(terminal.test);
       for (const case_ of terminal.cases) {
@@ -434,6 +454,10 @@ export function* eachTerminalOperand(terminal: Terminal): Iterable<Place> {
       yield terminal.test;
       break;
     }
+    case "branch": {
+      yield terminal.test;
+      break;
+    }
     case "switch": {
       yield terminal.test;
       for (const case_ of terminal.cases) {

compiler/forget/src/ReactiveScopes/BuildReactiveFunction.ts

@@ -360,6 +360,9 @@ class Driver {
         }
         break;
       }
+      case "branch": {
+        todo("Implement tree visitor for branch terminal");
+      }
       case "logical": {
         todo("Implement tree visitor for logical terminal");
       }

compiler/forget/src/Utils/VisualizeHIRMermaid.ts

@@ -77,6 +77,10 @@ function printTerminalLabel(terminal: Terminal): string {
       buffer.push(`If (${printPlace(terminal.test)})`);
       break;
     }
+    case "branch": {
+      buffer.push(`Branch (${printPlace(terminal.test)})`);
+      break;
+    }
     case "goto": {
       buffer.push("Goto");
       break;
@@ -130,6 +134,11 @@ function printTerminalArrows(blockId: BlockId, terminal: Terminal): string {
       }
       break;
     }
+    case "branch": {
+      buffer.push(printJumpArrow(blockId, terminal.consequent, "then"));
+      buffer.push(printJumpArrow(blockId, terminal.alternate, "else"));
+      break;
+    }
     case "logical": {
       buffer.push(printJumpArrow(blockId, terminal.test, "test"));
       buffer.push(printJumpArrow(blockId, terminal.fallthrough, "fallthrough"));