Auto merge of #45754 - scottmcm:checked-npot, r=dtolnay
Fix #18604: next_power_of_two should panic on overflow
Fixes https://github.com/rust-lang/rust/issues/18604
Is it possible to write a test for this? My experiments showed `x.py test` running in release mode, so my attempt at a `#[should_panic]` didn't work.