RFR: 8378793: Add ResolvedFieldEntry is_valid assert [v2]

[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 one additional commit since the last revision:

  Make assert_is_valid() only debug and only on ResolvedFieldEntry initialization.  Change fill_in to initialize flags also from fieldDescriptor, so set_flags and set_bytecodes can be private.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/29943/files
  - new: https://git.openjdk.org/jdk/pull/29943/files/9d56d096..1667f526

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29943&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29943&range=00-01

  Stats: 42 lines in 3 files changed: 20 ins; 20 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