RFR: 8348870: Optimization for DecimalDigits

[Shaojin Wen]

Expand DIGITS length to 128 and eliminate array bounds checking by using & 0x7F when accessing DIGITS

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Commit messages:
 - & 0x7f

Changes: https://git.openjdk.org/jdk/pull/23335/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23335&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8348870
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/23335.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23335/head:pull/23335

PR: https://git.openjdk.org/jdk/pull/23335