RFR: 8307190: Refactor ref_at methods in Constant Pool

[Matias Saavedra Silva]

In anticipation of [JDK-8301996](https://bugs.openjdk.org/browse/JDK-8301996), some of the accessors in constantpool.cpp need to be updated. The CPCache rework introduces multiple new meanings to the index argument passed to these functions, so they need to be restructured in a way that facilitates different paths depending on the input. For this enhancement, the bytecode is propagated by the callers to determine how to handle the index. Thanks to this and JDK-8307306, `bool uncached` is no longer needed in these functions.

Tests have been altered to suit the changes to JVMCI. Verified with tier1-5 tests.

-------------

Commit messages:
 - 8307190: Refactor ref_at methods in Constant Pool

Changes: https://git.openjdk.org/jdk/pull/13872/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13872&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307190
  Stats: 280 lines in 30 files changed: 39 ins; 68 del; 173 mod
  Patch: https://git.openjdk.org/jdk/pull/13872.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13872/head:pull/13872

PR: https://git.openjdk.org/jdk/pull/13872