RFR: JDK-8294902: Undefined Behavior in C2 regalloc with null references [v4]

[Andrew Haley]

> This patch fixes the remaining null pointer dereference bugs that I know of.
> 
> For the main bug, C2 was using a null reference to indicate an uninitialized `Node_List`. I replaced the null reference with a static sentinel.
> 
> I also turned on `-fsanitize=null` and found and fixed a bunch of other null pointer dereferences. With this,I have run a full bootstrap and tier1 tests with `-fsanitize=null` enabled.
> 
> I have checked that the code generated by GCC is not worse in any significant way, so I don't expect to see any performance regressions.
> 
> I'd like to enable `-fsanitize=null` in debug builds to prevent regressions in this area. What do you think?

Andrew Haley has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - Migrate postaloc.cpp migrated away from references to pointers when it comes to Node_List.
   
   Co-authored-by: Vladimir Ivanov <vaivanov at openjdk.org>
 - Merge from JDK head
 - Revert "Push ScopedValue tests"
   
   This reverts commit d298edfa9eda48ace9a27f83d38320fe6ba79e67.
 - Push ScopedValue tests
 - More
 - Next
 - Next
 - Next
 - Next

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Changes: https://git.openjdk.org/jdk/pull/10920/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10920&range=03
  Stats: 62 lines in 7 files changed: 24 ins; 1 del; 37 mod
  Patch: https://git.openjdk.org/jdk/pull/10920.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10920/head:pull/10920

PR: https://git.openjdk.org/jdk/pull/10920