RFR (S) 8231410: Shenandoah: clone barrier should use base pointer

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8231410

This is reproducible with x86_32, but the bug is generic, introduced by JDK-8231086.
ShenandoahRuntime::shenandoah_clone_barrier expects src to be the base pointer for the clone barrier
to act. C2, however, passes in the interior ptrs to the object contents. This leads to crashes, but
surprisingly only on x86_32.

Fix:
  https://cr.openjdk.java.net/~shade/8231410/webrev.02/

Testing: {x86_32, x86_64, aarch64} hotspot_gc_shenandoah; new regression test

-- 
Thanks,
-Aleksey