Retain original structure of labeled blocks

Handles some edge-cases where we previously flattened away some of the structure 
of a labeled block, instead ensuring that we retain the original shape. See the 
output. 

## Test Plan 

Tested on the internal app we're focused on (w useMemo inlining enabled), it 
works fine.

compiler/forget/src/HIR/MergeConsecutiveBlocks.ts

@@ -14,6 +14,7 @@ import {
   Instruction,
 } from "./HIR";
 import { markPredecessors, removeUnreachableFallthroughs } from "./HIRBuilder";
+import { mapOptionalFallthroughs } from "./visitors";
 
 /**
  * Merges sequences of blocks that will always execute consecutively —
@@ -86,6 +87,9 @@ export function mergeConsecutiveBlocks(fn: HIRFunction): void {
     fn.body.blocks.delete(block.id);
   }
   markPredecessors(fn.body);
+  for (const [, block] of fn.body.blocks) {
+    mapOptionalFallthroughs(block.terminal, (blockId) => merged.get(blockId));
+  }
   removeUnreachableFallthroughs(fn.body);
 }
 

compiler/forget/src/ReactiveScopes/BuildReactiveFunction.ts

@@ -573,15 +573,18 @@ class Driver {
       case "optional":
       case "ternary":
       case "logical": {
-        const fallthroughId = terminal.fallthrough;
-        invariant(
-          !this.cx.isScheduled(fallthroughId),
-          "Logical terminal fallthrough cannot have been scheduled"
-        );
-        const scheduleId = this.cx.schedule(fallthroughId, "if");
-        scheduleIds.push(scheduleId);
+        const fallthroughId =
+          terminal.fallthrough !== null &&
+          !this.cx.isScheduled(terminal.fallthrough)
+            ? terminal.fallthrough
+            : null;
+        if (fallthroughId !== null) {
+          const scheduleId = this.cx.schedule(fallthroughId, "if");
+          scheduleIds.push(scheduleId);
+        }
 
         const { place, value } = this.visitValueBlockTerminal(terminal);
+        this.cx.unscheduleAll(scheduleIds);
         blockValue.push({
           kind: "instruction",
           instruction: {
@@ -592,8 +595,9 @@ class Driver {
           },
         });
 
-        this.cx.unschedule(scheduleId);
-        this.visitBlock(this.cx.ir.blocks.get(fallthroughId)!, blockValue);
+        if (fallthroughId !== null) {
+          this.visitBlock(this.cx.ir.blocks.get(fallthroughId)!, blockValue);
+        }
         break;
       }
       case "goto": {

compiler/forget/src/__tests__/fixtures/compiler/dominator.expect.md

@@ -47,10 +47,11 @@ function Component(props) {
     x = 1;
   } else {
     if (props.b) {
-      x = 3;
     } else {
       break bb1;
     }
+
+    x = 3;
   }
   bb10: bb12: switch (props.c) {
     case "a": {

compiler/forget/src/__tests__/fixtures/compiler/early-return.expect.md

@@ -19,8 +19,6 @@ function MyApp(props) {
 function MyApp(props) {
   if (props.cond) {
     return;
-  } else {
-    return;
   }
 }
 

compiler/forget/src/__tests__/fixtures/compiler/useMemo-inverted-if.expect.md

@@ -23,11 +23,16 @@ function Component(props) {
 function Component(props) {
   let t17 = undefined;
   bb10: {
-    if (props.cond) {
-      t17 = props.b;
+    bb5: {
+      if (props.cond) {
+        break bb5;
+      }
+
+      t17 = props.a;
       break bb10;
     }
-    t17 = props.a;
+
+    t17 = props.b;
   }
   const x = t17;
   return x;