RFR: 8248188: Add IntrinsicCandidate and API for Base64 decoding [v7]

[CoreyAshford]

> This patch set encompasses the following commits:
> 
> - Adds a new HotSpot intrinsic candidate to the java.lang.Base64 class - decodeBlock(), and provides a flexible API for
>   the intrinsic.  The API is similar to the existing encodeBlock intrinsic.
> - Adds the code in HotSpot to check and martial the new intrinsic's arguments to the arch-specific intrinsic
>   implementation
> - Adds a Power64LE-specific implementation of the decodeBlock intrinsic.
> - Adds a JMH microbenchmark for both Base64 encoding and encoding.
> - Enhances the JTReg hotspot intrinsic "TestBase64.java" regression test to more fully test both decoding and encoding.

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

  CheckGraalIntrinsics.java: fix copy/paste error

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/293/files
  - new: https://git.openjdk.java.net/jdk/pull/293/files/dcd15d57..f93614dc

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=293&range=06
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=293&range=05-06

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/293.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/293/head:pull/293

PR: https://git.openjdk.java.net/jdk/pull/293