RFR: 8327964: Simplify BigInteger.implMultiplyToLen intrinsic

[Yudi Zheng]

Moving array construction within BigInteger.implMultiplyToLen intrinsic candidate to its caller simplifies the intrinsic implementation in JIT compiler.

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Commit messages:
 - Simplify BigInteger.implMultiplyToLen intrinsic

Changes: https://git.openjdk.org/jdk/pull/18226/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18226&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327964
  Stats: 53 lines in 2 files changed: 4 ins; 49 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18226.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18226/head:pull/18226

PR: https://git.openjdk.org/jdk/pull/18226