RFR: 8248188: Add HotSpotIntrinsicCandidate and API for Base64 decoding

[CoreyAshford]

On Tue, 22 Sep 2020 02:45:36 GMT, CoreyAshford <github.com+51754783+CoreyAshford at openjdk.org> wrote:

> 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.

This work is covered by an existing IBM <-> Oracle agreement

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