Auto merge of #96220 - RalfJung:scalar-no-padding, r=oli-obk

Auto merge of #96220 - RalfJung:scalar-no-padding, r=oli-obk

tighten sanity checks around Scalar and ScalarPair

While investigating https://github.com/rust-lang/rust/issues/96185 I noticed codegen has tighter sanity checks here than Miri does, so I added some more assertions. Strangely, some of them fail, so I also needed to add a HACK... that is probably worth looking into.

This does not fix that issue, but it changes the ICE messages, making it quite clear that we have a scalar whose size is not the same as that of the surrounding layout.

r? `@oli-obk`