Auto merge of #120405 - cjgillot:gvn-pointer, r=oli-obk

Fold pointer operations in GVN

This PR proposes 2 combinations of cast operations in MIR GVN:
- a chain of `PtrToPtr` or `MutToConstPointer` casts can be folded together into a single `PtrToPtr` cast;
- we attempt to evaluate more ptr ops when there is no provenance.

In particular, this allows to read from static slices.

This is not yet sufficient to see through slice operations that use `PtrComponents` (because that's a union), but still a step forward.

r? `@ghost`

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -415,7 +415,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         }
     }
 
-    fn unsize_into(
+    pub fn unsize_into(
         &mut self,
         src: &OpTy<'tcx, M::Provenance>,
         cast_ty: TyAndLayout<'tcx>,