fix corner case collapse_vars (#5569)

fixes #5568

lib/ast.js

@@ -141,6 +141,7 @@ var AST_Node = DEFNODE("Node", "start end", {
 }, null);
 
 DEF_BITPROPS(AST_Node, [
+    // AST_Node
     "_optimized",
     "_squeezed",
     // AST_Call
@@ -175,6 +176,8 @@ DEF_BITPROPS(AST_Node, [
     "pure",
     // AST_Assign
     "redundant",
+    // AST_Node
+    "single_use",
     // AST_ClassProperty
     "static",
     // AST_Call

lib/compress.js

@@ -2080,7 +2080,7 @@ Compressor.prototype.compress = function(node) {
                     return node;
                 }
                 // Skip transient nodes caused by single-use variable replacement
-                if (node.single_use && parent instanceof AST_VarDef && parent.value === node) return node;
+                if (node.single_use) return node;
                 // Replace variable with assignment when found
                 var hit_rhs;
                 if (!(node instanceof AST_SymbolDeclaration)