RFR: 8263041: Shenandoah: Cleanup C1 keep alive barrier check

[Zhengyu Gu]

Please review this cleanup. It appears that JDK-8233339 missed C1 change.

Test:
- [x] hotspot_gc_shenandoah

-------------

Commit messages:
 - JDK-8263041-c1-keepalive-check
 - update
 - JDK-8262852-varHandles-test

Changes: https://git.openjdk.java.net/jdk/pull/2834/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2834&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263041
  Stats: 20 lines in 2 files changed: 1 ins; 5 del; 14 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2834.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2834/head:pull/2834

PR: https://git.openjdk.java.net/jdk/pull/2834