Auto merge of #68493 - ecstatic-morse:enum-drop-elab-optim, r=<try>

[WIP] Don't `drop` enum if all fields are moved out of

Previously, drop elaboration is smart enough to realize when all fields of a struct or tuple are moved out of along a given code path and will remove drop terminators on that code path. However, if all fields of an *enum* with multiple variants are moved out of, drop terminators are still emitted. After this PR, drop elaboration will eliminate drops of an `Option` after the value in the `Some` variant is moved out of. `Option::unwrap` is an example of code whose MIR should be a bit better optimized as a result of this.

```rust
fn unwrap<T>(opt: Option<T>) -> T {
    match opt {
        Some(inner) => inner,
        None => panic!("unwrap"),
    }
}
```

When I say all fields of an enum, I mean all fields across all variants, so this optimization does not apply when, e.g., the value in `Result::Ok` is moved out of. To improve this case, we would need to incorporate information about the variant that is currently live into the `{Un,}InitializedPlaces` dataflow analyses. This is more involved, but I plan to give it a try.

Author: bors

src/librustc_mir/util/elaborate_drops.rs

@@ -472,7 +472,7 @@ where
                 }
                 let (normal, _) = self.drop_ladder(fields, succ, unwind);
                 normal_blocks.push(normal);
-            } else {
+            } else if !adt.variants[variant_index].fields.is_empty() {
                 have_otherwise = true;
             }
         }