Remove incorrect debug assertion in interpret

Cast type may be a subtype of the destination type. There is a later
assertion that they have the same layout.

src/librustc_mir/interpret/step.rs

@@ -259,8 +259,7 @@ fn eval_rvalue_into_place(
                 )?;
             }
 
-            Cast(kind, ref operand, cast_ty) => {
-                debug_assert_eq!(self.monomorphize(cast_ty)?, dest.layout.ty);
+            Cast(kind, ref operand, _) => {
                 let src = self.eval_operand(operand, None)?;
                 self.cast(src, kind, dest)?;
             }