fix corner case in side_effects (#5002)

fixes #5001

lib/compress.js

@@ -7596,11 +7596,22 @@ merge(Compressor.prototype, {
                             node.body = body;
                         }
                     } else if (body instanceof AST_Sequence) {
-                        var tail = body.tail_node();
+                        var exprs = body.expressions;
-                        if (tail instanceof AST_Await && is_primitive(compressor, tail.expression)) {
+                        for (var i = exprs.length; --i >= 0;) {
-                            tail = tail.expression.drop_side_effect_free(compressor);
+                            var tail = exprs[i];
-                            body.expressions.pop();
+                            if (!(tail instanceof AST_Await)) break;
-                            if (tail) body.expressions.push(tail);
+                            if (!is_primitive(compressor, tail.expression)) break;
+                            if (exprs[i] = tail.expression.drop_side_effect_free(compressor)) break;
+                        }
+                        switch (i) {
+                          case -1:
+                            return make_node(AST_EmptyStatement, node);
+                          case 0:
+                            node.body = exprs[0];
+                            break;
+                          default:
+                            exprs.length = i + 1;
+                            break;
                         }
                     }
                     return node;

test/compress/awaits.js

@@ -1758,3 +1758,25 @@ issue_4987: {
     ]
     node_version: ">=8"
 }
+
+issue_5001: {
+    options = {
+        awaits: true,
+        inline: true,
+        side_effects: true,
+    }
+    input: {
+        var a = 0;
+        (async function() {
+            a++ | await 42;
+        })();
+        console.log(a ? "PASS" : "FAIL");
+    }
+    expect: {
+        var a = 0;
+        void a++;
+        console.log(a ? "PASS" : "FAIL");
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}