RFR: 8337939: ZGC: Make assertions and checks less convoluted and explicit

[Joel Sikström]

There are currently cases where calls to type converters are made only to assert whether the conversion is reasonable or not and then discarding the result. For example, to_zaddress(...) is used to check if the pointer passed to it is a valid zaddress or not, whilst discarding the result of the conversion.

Additionally, a call like oopDesc::is_oop(to_oop(o)) is convoluted since a similar check to is_oop() is already done inside to_oop(), which should be a separate operation in its entirety.

Asserts/checks in affected places should be separated so that assertion/checking can be explicitly made and not done more than necessary.

Tested with tiers 1-7 on linux64 and linux64-debug.

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Commit messages:
 - Update zPage.inline.hpp
 - 8337939: ZGC: Make assertions and checks less convoluted and explicit

Changes: https://git.openjdk.org/jdk/pull/20478/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20478&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337939
  Stats: 57 lines in 10 files changed: 32 ins; 8 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/20478.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20478/head:pull/20478

PR: https://git.openjdk.org/jdk/pull/20478