RFR: Common TLS access to GC state, where possible

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/c2-common-gc-state/webrev.01/
(The initial version of this patch was drafted by Roland)

This patch bases on single GC state flag patch. This enables us to match that load at once, and
common all the loads of GC state between the safepoints, thus avoiding excess L1 cache accesses.
This covers for the cases where we cannot move the barriers themselves, and thus improves the
worst-case scenario.

It sure helps targeted back-to-back store benchmarks:

Benchmark                                    Mode  Cnt   Score    Error  Units

# default
BarriersMultiple.test                        avgt   15   5.935 ±  0.003  ns/op
BarriersMultiple.test:L1-dcache-loads        avgt    3  35.420 ±  2.116   #/op
BarriersMultiple.test:L1-dcache-stores       avgt    3   9.082 ±  0.603   #/op
BarriersMultiple.test:branches               avgt    3  18.187 ±  1.005   #/op
BarriersMultiple.test:cycles                 avgt    3  22.401 ±  1.249   #/op
BarriersMultiple.test:instructions           avgt    3  83.810 ±  4.297   #/op

# -XX:+ShenandoahCommonGCStateLoads
BarriersMultiple.test                        avgt   15   5.392 ±  0.116  ns/op
BarriersMultiple.test:L1-dcache-loads        avgt    3  26.302 ±  0.456   #/op  // -9!
BarriersMultiple.test:L1-dcache-stores       avgt    3   9.078 ±  1.174   #/op
BarriersMultiple.test:branches               avgt    3  18.218 ±  0.092   #/op
BarriersMultiple.test:cycles                 avgt    3  20.368 ±  3.023   #/op  // -2
BarriersMultiple.test:instructions           avgt    3  86.984 ±  1.127   #/op

...but comes with the caveat: the increased register pressure (?) seems to penalize some of the
bigger workloads. To avoid bitrot, and get the matchers for GC state loads into our codebase, I
propose pushing this under disabled experimental flag. New test validates the feature is not
completely broken.

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey