RFR: 8255614: Shenandoah: Consolidate/streamline runtime LRBs [v2]

[Roman Kennke]

> Currently, our various LRB entry points are a mess, and quite inefficient too.
> - We have three entry points, one is checking for null, and calls a non-null version, but that checks for null again
> - We don't have to check for null at all: it can be subsumed in the cset-check
> - The LRB resolves forwardee even though has_forwarded_objects() and in_cset() has not been checked
> - The LRB entry is not inlineable
> 
> The proposed change coalesces the 3 entries into one, moves it to shenandoahBarrierSet.inline.hpp and make it inlineable, rearranges the impl to allow cset-check to subsume the NULL-check. As a bonus, it pushes the NULL-check around keep-alive down after the (compile-time) check for weak-ref, so that this path becomes a no-op in the majority of cases.

Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:

  Add null-check

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/953/files
  - new: https://git.openjdk.java.net/jdk/pull/953/files/f2b87253..99db8d30

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=953&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=953&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/953.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/953/head:pull/953

PR: https://git.openjdk.java.net/jdk/pull/953