rustc_mir: use self.resolve instead of Instance::resolve where possible.

Author: eddyb

src/librustc_mir/interpret/cast.rs

@@ -39,12 +39,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                         if self.tcx.has_attr(def_id, sym::rustc_args_required_const) {
                             bug!("reifying a fn ptr that requires const arguments");
                         }
-                        let instance = ty::Instance::resolve(
-                            *self.tcx,
-                            self.param_env,
-                            def_id,
-                            substs,
-                        ).ok_or_else(|| err_inval!(TooGeneric))?;
+                        let instance = self.resolve(def_id, substs)?;
                         let fn_ptr = self.memory.create_fn_alloc(FnVal::Instance(instance));
                         self.write_scalar(Scalar::Ptr(fn_ptr.into()), dest)?;
                     }