Is it legal to reverse-engineer an integrated circuit (IC) belonging to a manufacturer
It's perfectly legal to open up a chip and try to figure out what it does and how it does it. Good luck. I have done that to competitors' equipment to see whether they infringed any of our patents (some did, some didn't).

It's illegal having done that to copy the chip and sell the results. Regardless of whether anything inside it is patented, the whole chip design, that specific implementation, will be subject to automatic copyright.