RFR: 8297561: Redundant index check in String.offsetByCodePoints() [v3]

[Sergey Tsypanov]

> `String.offsetByCodePoints()` delegates to `Character.offsetByCodePoints()` which in turn specifies the same exception thrown under the same conditions and the implementation does exactly the same checks. This means we can remove the check from `String.offsetByCodePoints()` and rely on the one of `Character.offsetByCodePoints()`.

Sergey Tsypanov has updated the pull request incrementally with two additional commits since the last revision:

 - Merge remote-tracking branch 'origin/JDK-8297561' into JDK-8297561
 - 8297561: Add copyright and change warmup&measurement time

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/11350/files
  - new: https://git.openjdk.org/jdk/pull/11350/files/1fdc0197..2d7b178c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11350&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11350&range=01-02

  Stats: 24 lines in 1 file changed: 22 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/11350.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11350/head:pull/11350

PR: https://git.openjdk.org/jdk/pull/11350