Compute LLVM argument indices correctly in face of padding

Closes #47278

src/librustc_trans/abi.rs

@@ -608,9 +608,6 @@ pub fn store(&self, bx: &Builder<'a, 'tcx>, val: ValueRef, dst: PlaceRef<'tcx>)
     }
 
     pub fn store_fn_arg(&self, bx: &Builder<'a, 'tcx>, idx: &mut usize, dst: PlaceRef<'tcx>) {
-        if self.pad.is_some() {
-            *idx += 1;
-        }
         let mut next = || {
             let val = llvm::get_param(bx.llfn(), *idx as c_uint);
             *idx += 1;

src/librustc_trans/mir/mod.rs

@@ -402,6 +402,9 @@ fn arg_local_refs<'a, 'tcx>(bx: &Builder<'a, 'tcx>,
             for i in 0..tupled_arg_tys.len() {
                 let arg = &fx.fn_ty.args[idx];
                 idx += 1;
+                if arg.pad.is_some() {
+                    llarg_idx += 1;
+                }
                 arg.store_fn_arg(bx, &mut llarg_idx, place.project_field(bx, i));
             }
 

src/test/codegen/issue-47278.rs

+// 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 {} }