The only real historical example that can be looked at as a parallel is when Reagan got shot in 1981. Yes, he did win the 1984 election in a landslide, but:

• He won the 1980 election in a landslide as well, so he was already super popular in the country
• He was the incumbent, and incumbents historically have done well in their re-election bids
• His opponent was Walter Mondale, a milquetoast neoliberal who was so boring it's surprising that his name didn't become synonymous with yawning.

In short, there's not really enough evidence to say definitively that an assassination attempt will bolster a candidate's chances at the polls.