RFR: 8013155: [pack200] improve performance of pack200

[Alexander Zuev]

Please review this fix which by fine-tuning of just one method gives up 
to 40% performance
improvement on packing and repacking of jar files.
Also removed unneeded local variable initialization from Code class - 
not for performance gain,
just to make world a better place.

http://cr.openjdk.java.net/~kizune/8013155/webrev.00