RFR: JDK-8217909: Make unused r12 register (without compressed oops) available to regalloc in C2

[Roman Kennke]

When running with compressed oops, the r12 register holds the heapbase,
and thus is not available to register allocation in C2. However, when
*not* running with compressed oops, it is still not available and
remains unused. It should be made available to register allocation in
this case.

This patch implements this by introducing with_r12 and no_r12 variants
of basically all register classes, and add dynamic reg classes to select
one or the other, based on current settings. I needed to add UseZGC in
those flags because ZGC asserts to not get r12 in its barriers. Not sure
that this is necessary.

Note that we might want to use the r12 register in Shenandoah later to
keep GC state. In this case, we'd need to add UseShenandoahGC or such to
the test. Do we want to abstract this whole check? Not sure that this is
possible/feasible to do in .ad though...

Bug:
https://bugs.openjdk.java.net/browse/JDK-8217909
Webrev:
http://cr.openjdk.java.net/~rkennke/JDK-8217909/webrev.00/

Testing: tier1 no regressions locally, eyeball generated code, yes it
does use r12 now.

Can I please get reviews?

Roman