Rollup merge of rust-lang#104357 - RalfJung:is-sized, r=cjgillot

add is_sized method on Abi and Layout, and use it

This avoids the double negation of `!is_unsized()` that we have quite a lot.

src/stacked_borrows/mod.rs

@@ -1053,7 +1053,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 // pointers we need to retag, so we can stop recursion early.
                 // This optimization is crucial for ZSTs, because they can contain way more fields
                 // than we can ever visit.
-                if !place.layout.is_unsized() && place.layout.size < self.ecx.pointer_size() {
+                if place.layout.is_sized() && place.layout.size < self.ecx.pointer_size() {
                     return Ok(());
                 }