RFR: 8272778: Consolidate is_instance and is_instance_inlined in java_lang_String

[Albert Mingkun Yang]

Simple change of merging `is_instance` and `is_instance_inlined`.

Test: build

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Commit messages:
 - inline

Changes: https://git.openjdk.java.net/jdk/pull/5201/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5201&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8272778
  Stats: 14 lines in 9 files changed: 0 ins; 4 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5201.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5201/head:pull/5201

PR: https://git.openjdk.java.net/jdk/pull/5201