mir: Don't load the discriminant, it's already in immediate form.

Author: eddyb

src/librustc_trans/trans/mir/rvalue.rs

@@ -248,15 +248,12 @@ impl<'bcx, 'tcx> MirContext<'bcx, 'tcx> {
                         let r_t_out = CastTy::from_ty(cast_ty).expect("bad output type for cast");
                         let ll_t_in = type_of::immediate_type_of(bcx.ccx(), operand.ty);
                         let ll_t_out = type_of::immediate_type_of(bcx.ccx(), cast_ty);
-                        let (llval, ll_t_in, signed) = if let CastTy::Int(IntTy::CEnum) = r_t_in {
+                        let llval = operand.immediate();
+                        let signed = if let CastTy::Int(IntTy::CEnum) = r_t_in {
                             let repr = adt::represent_type(bcx.ccx(), operand.ty);
-                            let llval = operand.immediate();
-                            let discr = bcx.with_block(|bcx| {
-                                adt::trans_get_discr(bcx, &repr, llval, None, true)
-                            });
-                            (discr, common::val_ty(discr), adt::is_discr_signed(&repr))
+                            adt::is_discr_signed(&repr)
                         } else {
-                            (operand.immediate(), ll_t_in, operand.ty.is_signed())
+                            operand.ty.is_signed()
                         };
 
                         let newval = match (r_t_in, r_t_out) {