RFR: 8281429: PhiNode::Value() is too conservative for tripcount of CountedLoop

[Roland Westrelin]

The type for the iv phi of a counted loop is computed from the types
of the phi on loop entry and the type of the limit from the exit
test. Because the exit test is applied to the iv after increment, the
type of the iv phi is at least one less than the limit (for a positive
stride, one more for a negative stride).

Also, for a stride whose absolute value is not 1 and constant init and
limit values, it's possible to compute accurately the iv phi type.

This change caused a few failures and I had to make a few adjustments
to loop opts code as well.

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Commit messages:
 - fix & test

Changes: https://git.openjdk.java.net/jdk/pull/7823/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7823&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8281429
  Stats: 152 lines in 5 files changed: 144 ins; 2 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7823.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7823/head:pull/7823

PR: https://git.openjdk.java.net/jdk/pull/7823