Fix handling of FFI arguments

r? @eddyb @nikomatsakis or whoever else.

cc @alexcrichton @rust-lang/core

The strategy employed here was to essentially change code we generate from

```llvm
  %s = alloca %S ; potentially smaller than argument, but never larger
  %1 = bitcast %S* %s to { i64, i64 }*
  store { i64, i64 } %0, { i64, i64 }* %1, align 4
```

to

```llvm
  %1 = alloca { i64, i64 } ; the copy of argument itself
  store { i64, i64 } %0, { i64, i64 }* %1, align 4
  %s = bitcast { i64, i64 }* %1 to %S* ; potentially truncate by casting to a pointer of smaller type.
```