RFR: 8273100: Improve AbstractStringBuilder.append(String) when using CompactStrings

[Claes Redestad]

Refactor to improve inlining, which helps some microbenchmarks exer StringBuilder.append(String)

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Commit messages:
 - 8273100: Improve AbstractStringBuilder.append(String) when using CompactStrings

Changes: https://git.openjdk.java.net/jdk/pull/5291/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5291&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273100
  Stats: 21 lines in 1 file changed: 11 ins; 2 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5291.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5291/head:pull/5291

PR: https://git.openjdk.java.net/jdk/pull/5291