RFC: shenandoah/jdk8u fails arraycopy verification after verification fix

[Roland Westrelin]

> The same change performs fine in shenandoah/jdk9 and shenandoah/jdk10. Reverting
> it fixes the test in shenandoah/jdk8u. What should we do about it?

The class hierarchy for cast nodes has changed since 8u. Replacing the

in->is_ConstraintCast()

test by

in->Opcode() == Op_CastPP || in->Opcode() == Op_CheckCastPP

should fix it.

Roland.