GVN: Elide more intermediate transmutes
We already skipped intermediate steps like `u32` or `i32` that support any (initialized) value.
This extends that to also allow skipping intermediate steps whose values are a superset of either the source or destination type. Most importantly, that means that `usize` → `NonZeroUsize` → `ptr::Alignment` and `ptr::Alignment` → `NonZeroUsize` → `usize` can skip the middle because `NonZeroUsize` is a superset of `Alignment`.
Then `Alignment::as_usize` is updated to take advantage of that and let us remove some more locals in a few places.
r? cjgillot
905b926967