RFR: 8254015: copy_to_survivor_space should use in-hand klass for scanning

[Kim Barrett]

Please review this small change to G1 evacuation.  When scanning the oops
in an instance, the existing code calls obj->oop_iterate_backward(closure).
That function gets the klass of obj and forwards to another function with
the obj, closure, and the just obtained klass.  But the caller of
oop_iterate_backward has already obtained the klass for other reasons.
Rather than refetching it (including dealing with
UseCompressedClassPointers), change to use a new oop_iterate_backward
overload taking the klass.  This saves a small amount of work for non-array
objects being evacuated.

Testing:
tier1
Generic performance testing didn't find any significant difference.
Looking at just GCBasher evacuation rate, there may to be a small
improvement, though it's hard to measure.

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Commit messages:
 - use in-hand klass

Changes: https://git.openjdk.java.net/jdk/pull/504/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=504&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8254015
  Stats: 11 lines in 3 files changed: 9 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/504.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/504/head:pull/504

PR: https://git.openjdk.java.net/jdk/pull/504