RFR: 8261905: Move implementation of OopStorage num_dead related functions

[Kim Barrett]

Please review this trivial change which just moves several functions to a
different location in the same file.  The old location is in the middle of
some unrelated functionality.

Testing:
mach5 tier1

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Commit messages:
 - move num_dead functions

Changes: https://git.openjdk.java.net/jdk/pull/2608/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2608&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261905
  Stats: 30 lines in 1 file changed: 15 ins; 15 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2608.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2608/head:pull/2608

PR: https://git.openjdk.java.net/jdk/pull/2608