RFR: 8335589: Fix -Wzero-as-null-pointer-constant warnings in IdealLoopTree ctor

[Kim Barrett]

Please review this trivial change to the IdealLoopTree constructor. The
initial values for some pointer-typed data members are changed from 0 to
nullptr. This removes ~275 -Wzero-as-null-pointer-constant when that warning
is enabled.

Testing: mach5 tier1

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Commit messages:
 - fix IdealLoopTree ctor

Changes: https://git.openjdk.org/jdk/pull/19995/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19995&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335589
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19995.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19995/head:pull/19995

PR: https://git.openjdk.org/jdk/pull/19995