Rollup merge of #105006 - RalfJung:scalar-pair-alignment, r=eddyb

stricter alignment enforcement for ScalarPair

`@eddyb` [indicated](#103926 (comment)) that alignment violating this check might be a bug. So let's see what the test suite says.

(Only the 2nd commit actually changes behavior... but I couldn't not do that other cleanup.^^)

Does the PR CI runner even enable debug assertions though...?

Author: matthiaskrgr