RFR: 8302163: Speed up various String comparison methods with ArraysSupport.mismatch [v2]

[Claes Redestad]

> We can improve various String methods such as `startsWith`, `endsWith` and `regionMatches` by leveraging the intrinsified mismatch methods in `ArraysSupport`.

Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:

  Clarify coder shift in startsWith

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12528/files
  - new: https://git.openjdk.org/jdk/pull/12528/files/6cac333d..b63954bf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12528&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12528&range=00-01

  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12528.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12528/head:pull/12528

PR: https://git.openjdk.org/jdk/pull/12528