LibJS: Implement 'this' in the bytecode VM

ThisExpression now emits a "ResolveThisBinding" bytecode op, which
simply loads the VM's current 'this' binding into the accumulator.

Userland/Libraries/LibJS/AST.h

@@ -1214,6 +1214,7 @@ public:
     }
     virtual Value execute(Interpreter&, GlobalObject&) const override;
     virtual void dump(int indent) const override;
+    virtual void generate_bytecode(Bytecode::Generator&) const override;
 };
 
 class CallExpression : public Expression {

Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp

@@ -1298,4 +1298,9 @@ void ClassDeclaration::generate_bytecode(Bytecode::Generator& generator) const
     generator.emit<Bytecode::Op::SetVariable>(generator.intern_string(m_class_expression.ptr()->name()));
 }
 
+void ThisExpression::generate_bytecode(Bytecode::Generator& generator) const
+{
+    generator.emit<Bytecode::Op::ResolveThisBinding>();
+}
+
 }

Userland/Libraries/LibJS/Bytecode/Instruction.h

@@ -61,6 +61,7 @@
     O(PushDeclarativeEnvironment)    \
     O(PutById)                       \
     O(PutByValue)                    \
+    O(ResolveThisBinding)            \
     O(Return)                        \
     O(RightShift)                    \
     O(SetVariable)                   \

Userland/Libraries/LibJS/Bytecode/Op.cpp

@@ -282,6 +282,11 @@ void Jump::execute_impl(Bytecode::Interpreter& interpreter) const
     interpreter.jump(*m_true_target);
 }
 
+void ResolveThisBinding::execute_impl(Bytecode::Interpreter& interpreter) const
+{
+    interpreter.accumulator() = interpreter.vm().resolve_this_binding(interpreter.global_object());
+}
+
 void Jump::replace_references_impl(BasicBlock const& from, BasicBlock const& to)
 {
     if (m_true_target.has_value() && &m_true_target->block() == &from)
@@ -785,4 +790,9 @@ String IteratorResultValue::to_string_impl(Executable const&) const
     return "IteratorResultValue";
 }
 
+String ResolveThisBinding::to_string_impl(Bytecode::Executable const&) const
+{
+    return "ResolveThisBinding"sv;
+}
+
 }

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -715,6 +715,18 @@ public:
     void replace_references_impl(BasicBlock const&, BasicBlock const&) { }
 };
 
+class ResolveThisBinding final : public Instruction {
+public:
+    explicit ResolveThisBinding()
+        : Instruction(Type::ResolveThisBinding)
+    {
+    }
+
+    void execute_impl(Bytecode::Interpreter&) const;
+    String to_string_impl(Bytecode::Executable const&) const;
+    void replace_references_impl(BasicBlock const&, BasicBlock const&) { }
+};
+
 }
 
 namespace JS::Bytecode {