RFR: 8378793: Add ResolvedFieldEntry is_valid assert [v3]

[Coleen Phillimore]

> Please review this small change to add an assert for values in ResolvedFieldEntry, which was useful for debugging at one time.
> Tested with tier1-3

Coleen Phillimore has updated the pull request incrementally with two additional commits since the last revision:

 - Fix assert string too.
 - Remove assert clause that can never be false.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29943/files
  - new: https://git.openjdk.org/jdk/pull/29943/files/1667f526..1b394bca

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29943&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29943&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/29943.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29943/head:pull/29943

PR: https://git.openjdk.org/jdk/pull/29943