LibJS: Support basic function calls in the bytecode world :^)

This patch adds the Call bytecode instruction which is emitted for the
CallExpression AST node.

It's pretty barebones and doesn't handle 'this' values properly, etc.
But it can perform basic function calls! :^)

Note that the called function will *not* execute as bytecode, but will
simply fall back into the old codepath and use the AST interpreter.

Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp

@@ -156,4 +156,25 @@ Optional<Bytecode::Register> MemberExpression::generate_bytecode(Bytecode::Gener
     }
 }
 
+Optional<Bytecode::Register> FunctionDeclaration::generate_bytecode(Bytecode::Generator&) const
+{
+    return {};
+}
+
+Optional<Bytecode::Register> CallExpression::generate_bytecode(Bytecode::Generator& generator) const
+{
+    auto callee_reg = m_callee->generate_bytecode(generator);
+
+    // FIXME: Load the correct 'this' value into 'this_reg'.
+    auto this_reg = generator.allocate_register();
+    generator.emit<Bytecode::Op::Load>(this_reg, js_undefined());
+
+    Vector<Bytecode::Register> argument_registers;
+    for (auto& arg : m_arguments)
+        argument_registers.append(*arg.value->generate_bytecode(generator));
+    auto dst_reg = generator.allocate_register();
+    generator.emit<Bytecode::Op::Call>(dst_reg, *callee_reg, this_reg, argument_registers);
+    return dst_reg;
+}
+
 }