An analyst upgrade highlights the case for investing in this exciting growth company.Machine vision company Cognex (CGNX -0.08%) recently received an analyst rating upgrade from hold to buy from HSBC. In addition, the price target was raised from $40 to $52. There's no doubt Cognex's fortunes are improving (the company returned to revenue growth in its last quarter). But is it enough to follow HSBC's lead and take a bullish opinion on the stock?Cognex's long-term growth prospectsI last discussed Cognex in F ...