RFR: 8278949: Cleanups for 8277850

[Roland Westrelin]

When 8277850 (C2: optimize mask checks in counted loops) was reviewed,
John made a number of comments and suggestions after the change was
integrated. This change includes all of his comments, extra tests to
cover all cases. I also moved the AndIL_add_shift_and_mask() call in
AndXNode::Ideal() up so the expression with a non constant mask can be
optimized as well.

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Commit messages:
 - whitespaces
 - John's comments

Changes: https://git.openjdk.java.net/jdk/pull/6876/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6876&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278949
  Stats: 333 lines in 3 files changed: 291 ins; 16 del; 26 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6876.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6876/head:pull/6876

PR: https://git.openjdk.java.net/jdk/pull/6876