[jdk11u-dev] RFR: 8268261: C2: assert(n != __null) failed: Bad immediate dominator info.

[Zhengyu Gu]

Not a clean backport due to context different, but easy to resolve manually.

Test:
- [x] New test failed w/o patch and passed with patch.

-------------

Commit messages:
 - Backport 4ed548b3eeffe8dfa601d4d0eeed2bfe359af965

Changes: https://git.openjdk.java.net/jdk11u-dev/pull/179/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk11u-dev&pr=179&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268261
  Stats: 75 lines in 2 files changed: 74 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk11u-dev/pull/179.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk11u-dev pull/179/head:pull/179

PR: https://git.openjdk.java.net/jdk11u-dev/pull/179