RFR: 8288968: C2: remove use of covariant returns in type.[ch]pp

[Roland Westrelin]

This removes some use of covariant returns that I added with 8275201
(C2: hide klass() accessor from TypeOopPtr and typeKlassPtr
subclasses).

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Commit messages:
 - covariant returns

Changes: https://git.openjdk.org/jdk/pull/9237/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9237&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288968
  Stats: 77 lines in 4 files changed: 0 ins; 0 del; 77 mod
  Patch: https://git.openjdk.org/jdk/pull/9237.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9237/head:pull/9237

PR: https://git.openjdk.org/jdk/pull/9237