RFR: 8277654: Shenandoah: Don't produce new memory state in C2 LRB runtime call

[Roman Kennke]

The runtime call in expanded Shenandoah LRB doesn't need to produce (and consume) raw memory state. I believe this is a left-over from when the LRB (or WB) allocated from TLABs, and would mess with TLAB pointers, when not ordered correctly. (Also, we used to require ordering with RBs back when we had them, but we already removed that memory dependency on -8 offset)

Testing:
 - [x] hotspot_gc_shenandoah
 - [x] specjvm -XX:+UseShenandoahGC
 - [x] tier1
 - [x] tier2
 - [x] tier3

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

Commit messages:
 - 8277654: Shenandoah: Don't produce new memory state in C2 LRB runtime call

Changes: https://git.openjdk.java.net/jdk/pull/6526/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6526&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8277654
  Stats: 32 lines in 2 files changed: 0 ins; 26 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6526.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6526/head:pull/6526

PR: https://git.openjdk.java.net/jdk/pull/6526