RFR: Misc cleanups

[Per Liden]

Spring cleaning continues! Here are five more patches that basically 
just removes currently unused code and removes some cosmetic compiler 
related changes that we've been carrying around in the master patch, but 
they don't really belong in the ZGC repo.

* ZGC: C2: Remove unused testL_reg_mem3 on x86
http://cr.openjdk.java.net/~pliden/zgc/remove_unused_testq/webrev.0

* ZGC: C1/C2: Remove unused infrastructure for R12-based load barrier 
support on x86
http://cr.openjdk.java.net/~pliden/zgc/remove_unused_infra_for_r12/webrev.0

* ZGC: C2: Remove unused variables in C2 code
http://cr.openjdk.java.net/~pliden/zgc/remove_unused_c2_variables/webrev.0

* ZGC: Remove left-overs in various compiler related files
http://cr.openjdk.java.net/~pliden/zgc/remove_misc_leftovers/webrev.0

* ZGC: Remove unused ZHeap::pagetable_addr() and friends
http://cr.openjdk.java.net/~pliden/zgc/remove_unused_pagetable_addr/webrev.0

/Per