RFR: 8232165: Reduce allocations in ValueStack copying constructor

[Claes Redestad]

Hi,

this patch optimizes C1 ValueStack copying, which reduces unnecessary
allocations and has a small but measurable impact on compilation speed.

Webrev: http://cr.openjdk.java.net/~redestad/8232165/open.00/
Bug:    https://bugs.openjdk.java.net/browse/JDK-8232165

Testing: tier1-2 (tier3-4 ongoing)

Thanks!

/Claes