Summary: Add a use of the node cx to avoid it being destroyed.
Backport from JDK-8217359: https://bugs.java.com/bugdatabase/view_bug.do?bug_id=8217359
This patch of OpenJDK is available here: https://hg.openjdk.java.net/jdk/jdk12/rev/44f41693631f

Test Plan: test/compiler/c2/Test8217359.java

Reviewed-by: luchsh, kuaiwei

Issue: https://github.com/alibaba/dragonwell8/issues/11