RFR: 8266845: Shenandoah: Simplify SBS::load_reference_barrier implementation [v3]

[Zhengyu Gu]

> Early return on null obj simplifies implementation and makes code more readable.
> 
> Test:
> - [x] hotspot_gc_shenandoah

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Removed unnecessary assert

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3956/files
  - new: https://git.openjdk.java.net/jdk/pull/3956/files/4a70d935..900185fb

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3956&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3956&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 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