RFR: JDK-8294902: Undefined Behavior in C2 regalloc with null references [v2]

[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 incrementally with one additional commit since the last revision:

  Push ScopedValue tests

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Changes:
  - all: https://git.openjdk.org/jdk/pull/10920/files
  - new: https://git.openjdk.org/jdk/pull/10920/files/5fe47361..d298edfa

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=10920&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=10920&range=00-01

  Stats: 1521 lines in 9 files changed: 1521 ins; 0 del; 0 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