RFR: 8266845: Shenandoah: Simplify SBS::load_reference_barrier implementation [v2]

[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:

  Aleksey's comments

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3956/files
  - new: https://git.openjdk.java.net/jdk/pull/3956/files/8ed81d6c..4a70d935

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3956&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3956&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 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