RFR: 8255534: Shenandoah: Fix CmpP optimization wrt native-LRB [v3]

[Roman Kennke]

> JDK-8254314 introduced the following code:
> 
>     if (in1->bottom_type() == TypePtr::NULL_PTR &&
>         (in1->Opcode() != Op_ShenandoahLoadReferenceBarrier || !((ShenandoahLoadReferenceBarrierNode*)in1)->is_native())) {
>       in2 = step_over_gc_barrier(in2);
>     }
> 
> However, the check for LRB and !native are the wrong way around: they should check if *in2* are not native LRB.
> 
> The bug is currently only observed in the conc-weakrefs branch, but may manifest (rarely) in mainline too. 
> 
> I am also putting in some refactoring to avoid duped code for good measure.
> 
> Testing: hotspot_gc_shenandoah (also in conc-weakrefs-branch where it manifests)

Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:

 - Inline (i.e. revert) maybe_step_over_cmpp_inputs()
 - Clarify comment in maybe_step_over_cmpp_inputs()

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/902/files
  - new: https://git.openjdk.java.net/jdk/pull/902/files/922d51ff..6169f948

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=902&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=902&range=01-02

  Stats: 24 lines in 2 files changed: 11 ins; 11 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/902.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/902/head:pull/902

PR: https://git.openjdk.java.net/jdk/pull/902