[C2] Backport JDK-8010500: Possible null pointer dereference at hotspot/src/share/vm/opto/loopnode.hpp

Summary:  Added NULL check for loopnode() in get_pre_loop_end()
Backport the fix for JDK-8010500 (https://hg.openjdk.java.net/jdk9/jdk9/hotspot/rev/4e7ee57b57bf) from OpenJDK.

Test Plan: none

Reviewed-by: kuaiwei, luchsh

Issue: https://github.com/alibaba/dragonwell8/issues/13