LibJS/Bytecode: Perform ToNumeric on accumulator before postfix inc/dec

This ensures we get the expected behavior of code like:

    let a = []
    let b = a++

(Where b should be 0, not [], because JavaScript.)

Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp

@@ -1966,6 +1966,7 @@ Bytecode::CodeGenerationErrorOr<void> UpdateExpression::generate_bytecode(Byteco
     Optional<Bytecode::Register> previous_value_for_postfix_reg;
     if (!m_prefixed) {
         previous_value_for_postfix_reg = generator.allocate_register();
+        generator.emit<Bytecode::Op::ToNumeric>();
         generator.emit<Bytecode::Op::Store>(*previous_value_for_postfix_reg);
     }