RFR: Demote ShenandoahAllocImplicitLive to diagnostic

[Aleksey Shipilev]

We will add it to sh/jdk9 backports too. This option is not user-settable:

$ hg qdiff
diff -r d2c26efa2d49 src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp
--- a/src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp	Thu Feb 15 15:53:21 2018 +0100
+++ b/src/hotspot/share/gc/shenandoah/shenandoah_globals.hpp	Thu Feb 15 17:54:02 2018 +0100
@@ -298,7 +298,7 @@
               "heap conditions, at the expense of higher perf overhead. "   \
               "Time is in milliseconds.")                                   \
                                                                             \
-  experimental(bool, ShenandoahAllocImplicitLive, true,                     \
+  diagnostic(bool, ShenandoahAllocImplicitLive, true,                       \
               "Treat (non-evac) allocations implicitely live")              \
                                                                             \
   diagnostic(bool, ShenandoahSATBBarrier, true,                             \

Testing: build

Thanks,
-Aleksey