RFR: 8260637: Shenandoah: assert(_base == Tuple) failure during C2 compilation

[Roland Westrelin]

Another shenandoah bug with a fix in shared code.

LRBRightAfterMemBar.test2() has 2 allocations that are non escaping
but non scalarizable. As a result, the null check for a3.f is
optimized out but the CastPP is left in the graph. That CastPP becomes
control dependent on the o2 == null check which is later hoisted out
of the loop. The CastPP is then right after the membar of the barrier = 0x42
volatile access but with an out of loop control. Because the node is
considered pinned by loopopts, it is assigned the membar as
control. The input of the CastPP is a shenandoah barrier that's
sandwiched between the membar and the CastPP and so expanded right
after the membar (that is between the membar and its control
projection). That causes the crash. I don't think cast nodes need to
be pinned so I propose that as a fix.

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Commit messages:
 - test & fix

Changes: https://git.openjdk.java.net/jdk/pull/2400/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2400&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260637
  Stats: 33 lines in 2 files changed: 27 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2400.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2400/head:pull/2400

PR: https://git.openjdk.java.net/jdk/pull/2400