RFR: 8282182: Document algorithm used to encode aarch64 logical immediate operands.

[Andrew Dinn]

This *documentation only* change explains how logical immediate mask values are derived from valid logical instruction operands. The encoding function is used to populate a sparse array that maps valid masks to a unique set of input operand values and a reverse lookup array that maps inputs to the associated mask.

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

Commit messages:
 - document algorithm aarch64 immediate encoding

Changes: https://git.openjdk.java.net/jdk/pull/7536/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7536&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282182
  Stats: 81 lines in 1 file changed: 74 ins; 1 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7536.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7536/head:pull/7536

PR: https://git.openjdk.java.net/jdk/pull/7536