RFR: 8275416: G1: remove unnecesary make_referent_alive in precleaning phase

[Albert Mingkun Yang]

Simple change of removing marking work in G1 precleaning. Evaluation using a contrived example shows ~20% reduction in precleaning. 

Test: hotspot_gc

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Commit messages:
 - g1-make-reference-alive

Changes: https://git.openjdk.java.net/jdk/pull/6003/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6003&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275416
  Stats: 42 lines in 3 files changed: 2 ins; 26 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6003.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6003/head:pull/6003

PR: https://git.openjdk.java.net/jdk/pull/6003