RFR: 8256730: Code that uses Object.checkIndex() range checks doesn't optimize well

[Roland Westrelin]

This was reported by Paul with the vector API. There are 2 issues:

- CastII nodes (added by Objects.checkIndex()) gets in the way of the
  pattern matching performed by range check elimination

- By transforming (CastII (AddI x y)) into (AddI (CastII x) (CastII y))
  some CastII can be eliminated which improves address computation code.

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Commit messages:
 - step over CastII in range checks & push CastII thru add

Changes: https://git.openjdk.java.net/jdk/pull/1342/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1342&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8256730
  Stats: 181 lines in 5 files changed: 116 ins; 55 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1342.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1342/head:pull/1342

PR: https://git.openjdk.java.net/jdk/pull/1342