[aarch64-port-dev ] bug fix for aarch64-port/jdk8u-shenandoah when Shenandoah is disabled

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/shenandoah/phi_has_only_data_users/webrev.00/

This is a fix for an issue that affects aarch64-port/jdk8u-shenandoah
even when shenandoah is disabled. That bug causes a debug VM to crash
with:

Internal Error at phaseX.cpp:985, pid=26493, tid=0x00007fd8c4199700
assert(false) failed: infinite loop in PhaseIterGVN::optimize

and a product build to consume a lot of memory.

Roland.