RFR: 8220501: Improve c1_ValueStack locks handling

[Claes Redestad]

Hi,

the _locks Values in ValueStack is often empty/unused, and allocating it
lazily is a small startup/warmup optimization.

Webrev: http://cr.openjdk.java.net/~redestad/8220501/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8220501

Testing: tier1-3

Thanks!

/Claes