Megacap stocks Apple IncAAPL.O, Tesla Inc TSLA.O and Microsoft CorpMSFT.O all rose solidly to give the biggest individual boosts to the S&P 500. By Lewis Krauskopf, Devik Jain and Anisha Sircar June 21 (Reuters) - Wall Street's major indexes jumped on Tuesday as investors scooped up shares of megacap growth and energy companies hammered last week on worries over a global economic downturn. Investors are trying to assess how far stocks can fall as they weigh risks to the economy with the Federal Reserve taking aggressive measures to try to tamp down surging inflation.