RFR: 8288992: AArch64: CMN should be handled the same way as CMP

[Andrew Haley]

At present, `cmp(r8, -1)` fails at compile time, but `cmn(r8, -1)` fails at runtime. We should fix cmn() to be the same as `cmp()`. 

After this change, it's much less likely that we'll be surprised by immediate overflows in `cmn()`.

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Commit messages:
 - 8288992: AArch64: CMN should be handled the same way as CMP

Changes: https://git.openjdk.org/jdk/pull/9246/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9246&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288992
  Stats: 9 lines in 2 files changed: 3 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/9246.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9246/head:pull/9246

PR: https://git.openjdk.org/jdk/pull/9246