borrowck::mir::dataflow: ignore unwind edges of empty drops

This avoids creating drop flags in many unnecessary situations.

Fixes #41110.

src/test/mir-opt/issue-41110.rs

@@ -0,0 +1,53 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// check that we don't emit multiple drop flags when they are not needed.
+
+fn main() {
+    let x = S.other(S.id());
+}
+
+pub fn test() {
+    let u = S;
+    let mut v = S;
+    drop(v);
+    v = u;
+}
+
+struct S;
+impl Drop for S {
+    fn drop(&mut self) {
+    }
+}
+
+impl S {
+    fn id(self) -> Self { self }
+    fn other(self, s: Self) {}
+}
+
+// END RUST SOURCE
+// START rustc.node4.ElaborateDrops.after.mir
+//    let mut _2: S;
+//    let mut _3: ();
+//    let mut _4: S;
+//    let mut _5: S;
+//    let mut _6: bool;
+//
+//    bb0: {
+// END rustc.node4.ElaborateDrops.after.mir
+// START rustc.node13.ElaborateDrops.after.mir
+//    let mut _2: ();
+//    let mut _4: ();
+//    let mut _5: S;
+//    let mut _6: S;
+//    let mut _7: bool;
+//
+//    bb0: {
+// END rustc.node13.ElaborateDrops.after.mir