RFR: 8313909: [JVMCI] assert(cp->tag_at(index).is_unresolved_klass()) in lookupKlassInPool

[Doug Simon]

This PR converts a very rare assertion failure into an exception throw that will a) be more informative about the unexpected value in debug build and b) bailout of a compilation in product builds.

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Commit messages:
 - be more defensive and informative in context of unexpected cp tag

Changes: https://git.openjdk.org/jdk/pull/20105/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20105&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313909
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20105.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20105/head:pull/20105

PR: https://git.openjdk.org/jdk/pull/20105