By Amy-Jo Crowley LONDON (Reuters) – WPP is considering the sale of its 40% stake in Kantar, a person with knowledge of the situation said, which could be valued at up to $8 billion including debt. The London-headquartered advertising company is working with an adviser and is considering selling the stake in the market research firm outright or selling it to co-shareholder Bain, the private equity fund, Bloomberg reported Wednesday. Shares in WPP turned higher after the report, rising 1.2%. A sale could help bolster confidence in WPP shares after the ad group in October cut its outlook for the…