Scaffolding for LoadGlobal instruction

Adds a new `LoadGlobal` InstructionValue variant which will be used to represent 
identifiers that refer to globals. We don't construct this value type yet.

compiler/forget/src/HIR/HIR.ts

@@ -442,6 +442,7 @@ export type InstructionData =
   | { kind: "ComputedStore"; object: Place; property: Place; value: Place }
   // load `object[index]` - like PropertyLoad but with a dynamic property
   | { kind: "ComputedLoad"; object: Place; property: Place }
+  | { kind: "LoadGlobal"; name: string }
   | FunctionExpression
   | {
       kind: "TaggedTemplateExpression";

compiler/forget/src/HIR/PrintHIR.ts

@@ -381,6 +381,10 @@ export function printInstructionValue(instrValue: ReactiveValue): string {
       value += instrValue.quasis.at(-1)!.raw + "`";
       break;
     }
+    case "LoadGlobal": {
+      value = `Global ${instrValue.name}`;
+      break;
+    }
     default: {
       assertExhaustive(
         instrValue,

compiler/forget/src/HIR/visitors.ts

@@ -131,6 +131,7 @@ export function* eachInstructionValueOperand(
       yield* instrValue.subexprs;
       break;
     }
+    case "LoadGlobal":
     case "UnsupportedNode":
     case "Primitive":
     case "JSXText": {
@@ -259,6 +260,7 @@ export function mapInstructionOperands(
       instrValue.subexprs = instrValue.subexprs.map(fn);
       break;
     }
+    case "LoadGlobal":
     case "UnsupportedNode":
     case "Primitive":
     case "JSXText": {

compiler/forget/src/Inference/InferReferenceEffects.ts

@@ -640,6 +640,7 @@ function inferBlock(
         effectKind = Effect.Read;
         break;
       }
+      case "LoadGlobal":
       case "JSXText":
       case "Primitive": {
         valueKind = ValueKind.Immutable;

compiler/forget/src/Optimization/DeadCodeElimination.ts

@@ -94,6 +94,7 @@ function pruneableValue(value: InstructionValue): boolean {
       // Potentially safe to prune, since they should just be creating new values
       return false;
     }
+    case "LoadGlobal":
     case "ArrayExpression":
     case "BinaryExpression":
     case "ComputedLoad":

compiler/forget/src/ReactiveScopes/CodegenReactiveFunction.ts

@@ -788,6 +788,10 @@ function codegenInstructionValue(
       );
       break;
     }
+    case "LoadGlobal": {
+      value = t.identifier(instrValue.name);
+      break;
+    }
     default: {
       assertExhaustive(
         instrValue,

compiler/forget/src/ReactiveScopes/InferReactiveScopeVariables.ts

@@ -168,6 +168,7 @@ function isMutable({ id }: Instruction, place: Place): boolean {
 
 function mayAllocate(value: InstructionValue): boolean {
   switch (value.kind) {
+    case "LoadGlobal":
     case "TypeCastExpression":
     case "BinaryExpression":
     case "Identifier":