RFR: 8325340: Add ASCII fast-path to Data-/ObjectInputStream.readUTF [v3]

[Claes Redestad]

> Adding a fast-path for ASCII-only modified UTF-8 strings deserialied via Data- and ObjectInputStream
> 
> Testing: tier1-3

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

  Revert "Refactor to use a shared impl for slow-path mod-UTF8 decoding"
  
  This reverts commit e6a8c3120b7f9c96a66ba40c27f8352735cdb588.

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17734/files
  - new: https://git.openjdk.org/jdk/pull/17734/files/e6a8c312..5655826b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17734&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17734&range=01-02

  Stats: 94 lines in 2 files changed: 46 ins; 13 del; 35 mod
  Patch: https://git.openjdk.org/jdk/pull/17734.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17734/head:pull/17734

PR: https://git.openjdk.org/jdk/pull/17734