librustc: Remove `simplify_type` and replace with `sizing_type_of`. rs=blocking-servo

`simplify_type` was bogus, as there was no way for it to handle enums
properly. It was also slow, because it created many Rust types at runtime. In
general creating Rust types during trans is a source of slowness, and I'd like
to avoid doing it as much as possible. (It is probably not possible to
eliminate it entirely, due to `subst`, but we should get rid of as much of it
as we can.) So this patch replaces `simplify_type` with `sizing_type_of`,
which creates a size-equivalent LLVM type directly without going through a
Rust type first.

Because this is causing an ICE in Servo, I'm rubber stamping it.