Auto merge of #29583 - dotdash:mir_small_agg, r=nikomatsakis

Fix handling of small aggregate function arguments and assignments of temporaries to lvalues.

src/test/run-pass/mir_small_agg_arg.rs

@@ -0,0 +1,19 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#![feature(rustc_attrs)]
+
+#[rustc_mir]
+fn foo((x, y): (i8, i8)) {
+}
+
+fn main() {
+    foo((0, 1));
+}