RFR: 8259020: null-check of g1 write_ref_field_pre_entry is not necessary

[Xin Liu]

orig is not null because G1BarrierSetC2 won't invoke write_ref_field_pre_entry 
if pre_val is NULL.

-------------

Commit messages:
 - 8259020: null-check of g1 write_ref_field_pre_entry is not necessary

Changes: https://git.openjdk.java.net/jdk/pull/1913/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1913&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259020
  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1913.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1913/head:pull/1913

PR: https://git.openjdk.java.net/jdk/pull/1913