RFR: 8292338: aarch64: Use cbnz instruction in gen_continuation_enter when possible

[Fei Yang]

This is a trivial improvement for gen_continuation_enter on aarch64.
On AAch64, we can combine Compare and Branch on Zero/Nonzero into CBZ/CBNZ to save one instruction.
Tier1 tested with fastdebug build on aarch64-linux-gnu.

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Commit messages:
 - 8292338: aarch64: Use cbnz instruction in gen_continuation_enter when possible

Changes: https://git.openjdk.org/jdk/pull/9872/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9872&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8292338
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9872.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9872/head:pull/9872

PR: https://git.openjdk.org/jdk/pull/9872