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

[CoreyAshford]

On Mon, 12 Oct 2020 21:38:36 GMT, CoreyAshford <github.com+51754783+CoreyAshford at openjdk.org> wrote:

>> Test java/util/Base64/TestBase64.java failed on Power9:
>> JavaTest Message: Test threw exception: java.lang.RuntimeException: Base64 decoding(String) failed!
>> Seed from RandomFactory = -8714459054005749075L
>> 
>> java.lang.RuntimeException: Base64 decoding(String) failed!
>> 	at TestBase64.checkEqual(TestBase64.java:523)
>> 	at TestBase64.test(TestBase64.java:185)
>> 	at TestBase64.main(TestBase64.java:61)
>
> This latest push passes the intrinsic regression test. I had run the intrinsic TestBase64 regression test on the
> previous push, but not the one in utils.  Interesting.  Somehow it didn't occur to me that there could be a problem
> there if the intrinsic TestBase64 test passed.  I will check into the other regression test.  Don't review this latest
> push just yet.

Ok, all is clear.  I just ran `jdk/java/util/Base64/TestBase64.java` which passes as well.  Please review again when
convenient.

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