RFR: 8299658: C1 compilation crashes in LinearScan::resolve_exception_edge

[Martin Doerr]

This is a quick fix for the C1 problem described in the JBS issue.
When we find an illegal operand (modelled by nullptr) while resolving an exception edge we can propagate this state to the phi function and skip the edge.

If somebody finds a better way to propagate the "illegal" state to the phi function, I can change or close this PR.

Please review. A nice regression test would be a good thing, but probably not easy to write.

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Commit messages:
 - 8299658: C1 compilation crashes in LinearScan::resolve_exception_edge

Changes: https://git.openjdk.org/jdk/pull/15348/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15348&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299658
  Stats: 8 lines in 1 file changed: 8 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15348.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15348/head:pull/15348

PR: https://git.openjdk.org/jdk/pull/15348