Bloomberg/Getty Images Gary Shilling is predicting a recession and a steeep stock plunge by year's end. The top economist said a downturn was almost inevitable, as consumer spending is likely to decline. He sees the S&P 500 dropping as much as 30% as stocks get punished for inflated valuations. Gary Shilling thinks there's almost nothing that can stop a US recession this year. The legendary economist and Merrill Lynch alum laid out a grim outlook for markets and the economy in an interview with Business Insider this week. In his view, it's almost inevitable that the US will tip into a recession this year, given ongoing vulnerabilities in multiple areas of the economy. He's also eyeing a big correction for stocks as valuations reach dizzying levels. Shilling said he believes the S&P 500 could end up tumbling by as much as 30%, with the bear-market decline potentially arriving by the end of the year.…