fix Rvalue::Use usage

Author: aapoalas

compiler/rustc_mir_transform/src/gvn.rs

@@ -1075,9 +1075,11 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
             }
             Rvalue::Reborrow(_, mutbl, place) => {
                 if mutbl == Mutability::Mut {
+                    // Note: this is adapted from simplify_aggregate.
                     let mut operand = Operand::Copy(place);
                     let val = self.simplify_operand(&mut operand, location);
-                    *rvalue = Rvalue::Use(operand);
+                    // FIXME(reborrow): Is it correct to make these retagging assignments?
+                    *rvalue = Rvalue::Use(Operand::Copy(place), WithRetag::Yes);
                     return val;
                 } else {
                     // FIXME(reborrow): CoerceShared should perform effectively a copy followed by a