RFR: 8299155: C2: SubTypeCheckNode::verify() should not produce dependencies / oop pool entries

[Roland Westrelin]

This patch simply guarantees a new LoadKlassNode is not created (and
processed by igvn) unless it's really needed.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/12383/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12383&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299155
  Stats: 27 lines in 2 files changed: 17 ins; 9 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12383.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12383/head:pull/12383

PR: https://git.openjdk.org/jdk/pull/12383