Fix underflow when calculating the number of no-op jumps folded

When removing unwinds to no-op blocks and folding jumps to no-op blocks,
remove the unwind target first. Otherwise we cannot determine if target
has been already folded or not.

Previous implementation incorrectly assumed that all resume targets had
been folded already, occasionally resulting in an underflow:

remove_noop_landing_pads: removed 18446744073709551613 jumps and 3 landing pads

Author: tmiasko

compiler/rustc_mir/src/transform/remove_noop_landing_pads.rs

@@ -102,6 +102,16 @@ impl RemoveNoopLandingPads {
         let postorder: Vec<_> = traversal::postorder(body).map(|(bb, _)| bb).collect();
         for bb in postorder {
             debug!("  processing {:?}", bb);
+            if let Some(unwind) = body[bb].terminator_mut().unwind_mut() {
+                if let Some(unwind_bb) = *unwind {
+                    if nop_landing_pads.contains(unwind_bb) {
+                        debug!("    removing noop landing pad");
+                        landing_pads_removed += 1;
+                        *unwind = None;
+                    }
+                }
+            }
+
             for target in body[bb].terminator_mut().successors_mut() {
                 if *target != resume_block && nop_landing_pads.contains(*target) {
                     debug!("    folding noop jump to {:?} to resume block", target);
@@ -110,15 +120,6 @@ impl RemoveNoopLandingPads {
                 }
             }
 
-            if let Some(unwind) = body[bb].terminator_mut().unwind_mut() {
-                if *unwind == Some(resume_block) {
-                    debug!("    removing noop landing pad");
-                    jumps_folded -= 1;
-                    landing_pads_removed += 1;
-                    *unwind = None;
-                }
-            }
-
             let is_nop_landing_pad = self.is_nop_landing_pad(bb, body, &nop_landing_pads);
             if is_nop_landing_pad {
                 nop_landing_pads.insert(bb);