RFR: 8282182: Document algorithm used to encode aarch64 logical immediate operands. [v2]

[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.

Andrew Dinn has updated the pull request incrementally with two additional commits since the last revision:

 - more logical flow to explanation
 - address initial reviewer feedback

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7536/files
  - new: https://git.openjdk.java.net/jdk/pull/7536/files/d664a793..737c1160

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7536&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7536&range=00-01

  Stats: 29 lines in 1 file changed: 7 ins; 3 del; 19 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