enums with SIMD vectors in them don't work

[jld]

pub fn main() { let _o = None::<std::unstable::simd::i32x4>; }

task 'rustc' failed at 'Unsupported enum alignment: 16u64', /home/jld/src/rust/src/librustc/middle/trans/adt.rs:409

Known and explained in a comment in the source, but I didn't realize when I wrote it that we'd exposed vectors already.

Should be a simple matter of adding more cases to the match align { ... } part.

cc @huonw