RFR: 8278471: Remove unreached rules in AddNode::IdealIL [v4]

[Zhiqiang Zang]

> Reorder optimizations in addnode so special cases appear before general cases; otherwise the special cases would be never covered.
> 
> `(a - b) + (c - d)` subsumes both `(a - b) + (b - c)` and `(a - b) + (c - a)`. Therefore `(a - b) + (b - c)` and `(a - b) + (c - a)` have to be placed before `(a - b) + (c - d)` so that they can work.

Zhiqiang Zang has updated the pull request incrementally with two additional commits since the last revision:

 - Add missing optimizations "(A+X) - (X+B)" and "(X+A) - (B+X)" for long type.
 - Include an ir test to verify the removed optimizations really happended already.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6752/files
  - new: https://git.openjdk.java.net/jdk/pull/6752/files/d4d2b180..841d404f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6752&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6752&range=02-03

  Stats: 86 lines in 3 files changed: 86 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6752.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6752/head:pull/6752

PR: https://git.openjdk.java.net/jdk/pull/6752