emit "align 1" metadata on loads/stores of packed structs

According to the LLVM reference:
> A value of 0 or an omitted align argument means that the operation has
the ABI alignment for the target.

So loads/stores of fields of packed structs need to have their align set
to 1. Implement that by tracking the alignment of `LvalueRef`s.

Fixes #39376.

src/test/codegen/packed.rs

@@ -0,0 +1,29 @@
+// Copyright 2016 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.
+
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+
+#[repr(packed)]
+pub struct Packed {
+    dealign: u8,
+    data: u32
+}
+
+// CHECK-LABEL: @write_pkd
+#[no_mangle]
+pub fn write_pkd(pkd: &mut Packed) -> u32 {
+// CHECK: %{{.*}} = load i32, i32* %{{.*}}, align 1
+// CHECK: store i32 42, i32* %{{.*}}, align 1
+    let result = pkd.data;
+    pkd.data = 42;
+    result
+}