RFR: 8285001: Simplify StringLatin1.regionMatches

[Claes Redestad]

There is no pair of character values in the latin1 range (0-255) that will make the condition at line 401 in `StringLatin1.java` true, so this test can be removed.

Added a test and a microbenchmark (which as expected sees a few ns/op improvement from this change). Took the opportunity to tune the default settings for the micro to reduce runtime from 30+ minutes to 3 with no discernible degradation of quality.

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Commit messages:
 - Add tests and micro
 - Simplify latin1 to latin1 locale-independent comparisons in StringLatin1.regionMatchesCI

Changes: https://git.openjdk.java.net/jdk/pull/8292/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8292&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8285001
  Stats: 94 lines in 3 files changed: 88 ins; 3 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8292.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8292/head:pull/8292

PR: https://git.openjdk.java.net/jdk/pull/8292