Rollup merge of #107271 - Zeegomo:drop-rmw, r=oli-obk

Treat Drop as a rmw operation

Previously, a Drop terminator was considered a move in MIR. This commit changes the behavior to only treat Drop as a mutable access to the dropped place.

In order for this change to be correct, we need to guarantee that

1.  A dropped value won't be used again
   2.  Places that appear in a drop won't be used again before a
     subsequent initialization.

We can ensure this to be correct at MIR construction because Drop will only be emitted when a variable goes out of scope, thus having:
*   (1) as there is no way of reaching the old value. drop-elaboration
     will also remove any uninitialized drop.
 * (2) as the place can't be named following the end of the scope.

However, the initialization status, previously tracked by moves, should also be tied to the execution of a Drop, hence the additional logic in the dataflow analyses.

From discussion in [this thread](https://rust-lang.zulipchat.com/#narrow/stream/233931-t-compiler.2Fmajor-changes/topic/.60DROP.60.20to.20.60DROP_IF.60.20compiler-team.23558), originating from https://github.com/rust-lang/compiler-team/issues/558.
See also https://github.com/rust-lang/rust/pull/104488#discussion_r1085556010

compiler/rustc_mir_dataflow/src/move_paths/builder.rs

@@ -376,7 +376,8 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
             | TerminatorKind::Resume
             | TerminatorKind::Abort
             | TerminatorKind::GeneratorDrop
-            | TerminatorKind::Unreachable => {}
+            | TerminatorKind::Unreachable
+            | TerminatorKind::Drop { .. } => {}
 
             TerminatorKind::Assert { ref cond, .. } => {
                 self.gather_operand(cond);
@@ -391,10 +392,6 @@ impl<'b, 'a, 'tcx> Gatherer<'b, 'a, 'tcx> {
                 self.create_move_path(place);
                 self.gather_init(place.as_ref(), InitKind::Deep);
             }
-
-            TerminatorKind::Drop { place, target: _, unwind: _ } => {
-                self.gather_move(place);
-            }
             TerminatorKind::DropAndReplace { place, ref value, .. } => {
                 self.create_move_path(place);
                 self.gather_operand(value);