RFR: 8248188: Add IntrinsicCandidate and API for Base64 decoding [v4]

[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 two additional commits since the last revision:

 - TestBase64.java: fix comment to correctly reflect actual intrinsic names.
   
   The intrinsic names that are visible with -XX:+PrintCompilation are encode
   and decode, rather than encodeBlock and decodeBlock.
 - stubGenerator_ppc.cpp: fix regression caused by change to using loop counter
   
   My original fix didn't account for the case where sl < block_size.  In the
   event sl < block_size, the shifted sl will become zero, so it should
   jump to the code that computes how much data was processed - 0 - and return.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/293/files
  - new: https://git.openjdk.java.net/jdk/pull/293/files/8932c233..164fa2a9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=293&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=293&range=02-03

  Stats: 13 lines in 2 files changed: 7 ins; 4 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