fix FIXME(#6449) in #[derive(PartialOrd, Ord)]

fix FIXME(#6449) in #[derive(PartialOrd, Ord)]

This replaces some `if`s with `match`es. This was originally not possible
because using a global path in a match statement caused a "non-constant
path in constant expr" ICE. The issue is long since closed, though you still
hit it (as an error now, not an ICE) if you try to generate match patterns
using pat_lit(expr_path). But it works when constructing the patterns more
carefully.