RFR: 8283008: KRegister documentation out of date

[Thomas Schatzl]

Hi all,

  can I have reviews for this cleanup of `KRegister(Impl)` documentation, currently being a mix of copy&paste from `XMMRegister` and obsolete MMX support.

Testing: local compilation

Thanks,
  Thomas

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Commit messages:
 - Initial change

Changes: https://git.openjdk.java.net/jdk/pull/7784/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7784&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283008
  Stats: 7 lines in 1 file changed: 0 ins; 4 del; 3 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7784.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7784/head:pull/7784

PR: https://git.openjdk.java.net/jdk/pull/7784