RFR(S): 8195001: [MVT] Avoid allocations of default value types

[Tobias Hartmann]

Hi,

please review the following enhancement that I had in my queue for a while:
https://bugs.openjdk.java.net/browse/JDK-8195001
http://cr.openjdk.java.net/~thartmann/8195001/webrev.00/

C2 now detects default value types more reliably and aggressively removes their allocations by replacing them with the
pre-allocated default oop (see test35/test36). All tests pass.

Thanks,
Tobias