fix corner case in hoist_props (#5223)

fixes #5222

lib/ast.js

@@ -1537,6 +1537,12 @@ var AST_Assign = DEFNODE("Assign", null, {
                     throw new Error("left must be assignable: " + node.TYPE);
                 }
             });
+        } else if (!(this.left instanceof AST_Infinity
+            || this.left instanceof AST_NaN
+            || this.left instanceof AST_PropAccess && !this.left.optional
+            || this.left instanceof AST_SymbolRef
+            || this.left instanceof AST_Undefined)) {
+            throw new Error("left must be assignable");
         }
     },
 }, AST_Binary);

lib/compress.js

@@ -7795,6 +7795,7 @@ Compressor.prototype.compress = function(node) {
                 var defs = defs_by_id[node.definition().id];
                 if (!defs) return;
                 if (node.fixed_value() !== defs.value) return;
+                if (is_lhs(node, this.parent())) return;
                 return make_node(AST_Object, node, { properties: [] });
             }
         }));