RFR: 8255400: Shenandoah: C2 failures after JDK-8255000

[Roland Westrelin]

At barrier expansion time, the IR graph may contain a Halt node whose
control is a region. In that case, code that wires raw memory creates
a memory Phi at the region. But that Phi has no use because the Halt
node doesn't consume any memory. That dead Phi causes the assert to
trigger. I propose some adjustments so a Phi is not created in that
case.

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Commit messages:
 - jcheck
 - more test
 - fix
 - test

Changes: https://git.openjdk.java.net/jdk/pull/1000/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1000&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8255400
  Stats: 87 lines in 2 files changed: 81 ins; 0 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1000.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1000/head:pull/1000

PR: https://git.openjdk.java.net/jdk/pull/1000