RFR (XS): 8008938:TieredCompilation should be default

[Vladimir Kozlov]

http://cr.openjdk.java.net/~kvn/8008938/webrev/

Backout 8005811 changes which switched TieredCompilation off until we 
resolved some issues with CodeCache.
We recently pushed several CodeCache changes to address those issues. So 
we can turn Tiered on again.

Tested with jprt and we do regular Nightly testing with +TieredCompilation.

Thanks,
Vladimir