RFR: 8282429: StringBuilder/StringBuffer.toString() skip compressing for UTF16 strings

[Xin Liu]

If AbstractStringBuilder only grow, the inflated value which is encoded in UTF16 can't be compressed. 
toString() can skip compression in this case. This can save an ArrayAllocation in StringUTF16::compress.

java.io.BufferedRead::readLine() is a case that StringBuilder grows only.

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Commit messages:
 - Add a microbenchmark.
 - 8282429:  StringBuilder/StringBuffer.toString() skip compressing for UTF16 strings

Changes: https://git.openjdk.java.net/jdk/pull/7671/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7671&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282429
  Stats: 44 lines in 5 files changed: 37 ins; 3 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7671.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7671/head:pull/7671

PR: https://git.openjdk.java.net/jdk/pull/7671