[hir][be] Use a Map to store JsxExpression.props

Semantically this seems like a better fit as we're using Map like methods to 
iterate and update values anyway.

compiler/forget/src/HIR/BuildHIR.ts

@@ -860,7 +860,7 @@ function lowerExpression(
       const children = expr
         .get("children")
         .map((child) => lowerJsxElement(builder, child));
-      const props: { [prop: string]: Place } = {};
+      const props: Map<string, Place> = new Map();
       opening.get("attributes").forEach((attribute) => {
         todoInvariant(attribute.isJSXAttribute(), "handle spread attributes");
         const name = attribute.get("name");
@@ -882,7 +882,7 @@ function lowerExpression(
           value = lowerExpressionToPlace(builder, expression);
         }
         const prop: string = name.node.name;
-        props[prop] = value;
+        props.set(prop, value);
       });
       return {
         kind: "JsxExpression",

compiler/forget/src/HIR/Codegen.ts

@@ -215,7 +215,7 @@ function writeInstr(cx: Context, instr: Instruction, body: Array<t.Statement>) {
     }
     case "JsxExpression": {
       const attributes: Array<t.JSXAttribute> = [];
-      for (const [prop, value] of Object.entries(instrValue.props)) {
+      for (const [prop, value] of instrValue.props) {
         attributes.push(
           t.jsxAttribute(
             t.jsxIdentifier(prop),

compiler/forget/src/HIR/HIR.ts

@@ -185,7 +185,7 @@ export type InstructionData =
   | {
       kind: "JsxExpression";
       tag: Place;
-      props: { [prop: string]: Place };
+      props: Map<string, Place>;
       children: Array<Place> | null; // null === no children
     }
   | {

compiler/forget/src/HIR/InferReferenceEffects.ts

@@ -546,7 +546,7 @@ function inferBlock(env: Environment, block: BasicBlock) {
       case "JsxExpression": {
         valueKind = ValueKind.Frozen;
         env.reference(instrValue.tag, Effect.Freeze);
-        for (const [_prop, value] of Object.entries(instrValue.props)) {
+        for (const [_prop, value] of instrValue.props) {
           env.reference(value, Effect.Freeze);
         }
         if (instrValue.children !== null) {

compiler/forget/src/HIR/PrintHIR.ts

@@ -191,7 +191,7 @@ function printInstructionValue(instrValue: InstructionValue): string {
     }
     case "JsxExpression": {
       const propItems = [];
-      for (const [prop, value] of Object.entries(instrValue.props)) {
+      for (const [prop, value] of instrValue.props) {
         propItems.push(`${prop}={${printPlace(value)}}`);
       }
       const props = propItems.length !== 0 ? " " + propItems.join(" ") : "";