Rollup merge of #119835 - Nadrieril:simplify-empty-logic, r=compiler-errors

Exhaustiveness: simplify empty pattern logic

The logic that handles empty patterns had gotten quite convoluted. This PR simplifies it a lot. I tried to make the logic as easy as possible to follow; this only does logically equivalent changes.

The first commit is a drive-by comment clarification that was requested after another PR a while back.

r? `@compiler-errors`

Author: matthiaskrgr