RFR(S): 8190458: [MVT] C2 should use pre-allocated default value types

[Tobias Hartmann]

Hi,

please review the following patch:
https://bugs.openjdk.java.net/browse/JDK-8190458
http://cr.openjdk.java.net/~thartmann/8190458/webrev.00/

This is based on top of Fred's patch for 8190432 [1]. C2 should load and re-use the default oop from the java mirror 
instead of allocating a default value type.

All tests pass.

Thanks,
Tobias

[1] http://mail.openjdk.java.net/pipermail/valhalla-dev/2017-October/003500.html