RFR: 8280123: C2: Infinite loop in CMoveINode::Ideal during IGVN

[Vladimir Ivanov]

There's a discrepancy between `CMoveNode::Ideal()` and `CMoveINode::Ideal()` in the way how constants are detected:
the former looks for `Con` nodes (`in(IfFalse)->is_Con()`) while the latter checks node types (e.g., `phase->type(in(IfFalse)) == TypeInt::ZERO`). It leads to infinite loop during IGVN (where  `CMoveNode::Ideal()` and `CMoveINode::Ideal()` repeatedly rotate `CMoveI` node) because `CMoveNode::Ideal()` don't consider `CastII` to a constant as a constant while `CMoveINode::Ideal()` does.

The fix migrates `CMoveNode::Ideal()` away from `Node::is_Con()` to node types.

Testing: hs-tier1 - hs-tier4

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Commit messages:
 - 8280123: C2: Infinite loop in CMoveINode::Ideal during IGVN

Changes: https://git.openjdk.java.net/jdk/pull/7133/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7133&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280123
  Stats: 87 lines in 2 files changed: 70 ins; 4 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7133.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7133/head:pull/7133

PR: https://git.openjdk.java.net/jdk/pull/7133