Auto merge of #47401 - rkruppe:issue-47278, r=eddyb

Compute LLVM argument indices correctly in face of padding

Closes #47278

r? @eddyb

src/test/codegen/issue-47278.rs

@@ -0,0 +1,19 @@
+// Copyright 2018 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.
+
+// -C no-prepopulate-passes
+#![crate_type="staticlib"]
+
+#[repr(C)]
+pub struct Foo(u64);
+
+// CHECK: define {{.*}} @foo(
+#[no_mangle]
+pub extern fn foo(_: Foo) -> Foo { loop {} }