simplify_try: address some of eddyb's comments

src/librustc_mir/transform/simplify_try.rs

@@ -52,6 +52,8 @@ impl<'tcx> MirPass<'tcx> for SimplifyArmIdentity {
                 Some(x) => x,
             };
             if local_tmp_s0 != local_tmp_s1
+                // Avoid moving into ourselves.
+                || local_0 == local_1
                 // The field-and-variant information match up.
                 || vf_s0 != vf_s1
                 // Source and target locals have the same type.
@@ -64,6 +66,7 @@ impl<'tcx> MirPass<'tcx> for SimplifyArmIdentity {
             }
 
             // Right shape; transform!
+            s0.source_info = s2.source_info;
             match &mut s0.kind {
                 StatementKind::Assign(box (place, rvalue)) => {
                     *place = local_0.into();