Auto merge of #151622 - scottmcm:elide-more-transmutes, r=cjgillot

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

Author: bors