remove recursive collapse_single_use_vars()

- let `OPT(AST_Node)` do their job

lib/compress.js

@@ -647,11 +647,6 @@ merge(Compressor.prototype, {
             for (var stat_index = statements.length; --stat_index >= 0;) {
                 var stat = statements[stat_index];
 
-                // Process child blocks of statement if present.
-                [stat, stat.body, stat.alternative, stat.bcatch, stat.bfinally].forEach(function(node) {
-                    node && node.body && collapse_single_use_vars(node.body, compressor);
-                });
-
                 // The variable definition must precede a statement.
                 if (stat_index <= 0) break;
                 var prev_stat_index = stat_index - 1;