RFR: 8266845: Shenandoah: Simplify SBS::load_reference_barrier implementation

[Zhengyu Gu]

Early return on null obj simplifies implementation and makes code more readable.

Test:
- [x] hotspot_gc_shenandoah

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Commit messages:
 - update
 - Merge branch 'master' into lrb-null-obj
 - v1

Changes: https://git.openjdk.java.net/jdk/pull/3956/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3956&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266845
  Stats: 8 lines in 1 file changed: 4 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3956.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3956/head:pull/3956

PR: https://git.openjdk.java.net/jdk/pull/3956