RFR: 8320759: Creation of random BigIntegers can be made faster

[fabioromano1]

A faster and simpler way to generate random BigIntegers, avoiding eventually trimming of leading zeros in magnitude array.

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Commit messages:
 - Optimized randomBits(int, Random)
 - Removed trailing whitespaces
 - Correct randomBits(int, Random)
 - Update BigInteger.java
 - Merge branch 'openjdk:master' into patch-4
 - Merge branch 'patch-4' of https://github.com/fabioromano1/jdk into patch-4
 - Merge branch 'openjdk:master' into patch-4
 - An optimization
 - Correct BigInteger(int , Random)
 - Made random generation of BigIntegers faster

Changes: https://git.openjdk.org/jdk/pull/16817/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16817&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320759
  Stats: 39 lines in 1 file changed: 16 ins; 8 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/16817.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16817/head:pull/16817

PR: https://git.openjdk.org/jdk/pull/16817