RFR: 8356891: Some code simplifications for basic BigIntegers' bit operations

[fabioromano1]

Some changes in `Biginteger`s' bit operations that increase the code readability and slightly optimize the execution time.

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Commit messages:
 - More clear comment
 - An optimization
 - Code simplification
 - Code simplification
 - Avoiding redundant code
 - Code simplification
 - Code simplifications for BigIntegers' bit operations

Changes: https://git.openjdk.org/jdk/pull/25166/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25166&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8356891
  Stats: 83 lines in 1 file changed: 14 ins; 36 del; 33 mod
  Patch: https://git.openjdk.org/jdk/pull/25166.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25166/head:pull/25166

PR: https://git.openjdk.org/jdk/pull/25166