Rollup merge of #71479 - RalfJung:scalar-null-ptr, r=jonas-schievink

add back Scalar::null_ptr

We were a bit overeager with removing this in #71005, Miri uses this function quite a bit.

The important part is that the `Place::null` methods are gone. :)
r? @jonas-schievink @oli-obk

Fixes #71474

src/librustc_middle/mir/interpret/value.rs

@@ -188,6 +188,11 @@ impl<'tcx, Tag> Scalar<Tag> {
         }
     }
 
+    #[inline]
+    pub fn null_ptr(cx: &impl HasDataLayout) -> Self {
+        Scalar::Raw { data: 0, size: cx.data_layout().pointer_size.bytes() as u8 }
+    }
+
     #[inline]
     pub fn zst() -> Self {
         Scalar::Raw { data: 0, size: 0 }