[11u] RFR 8250825: C2 crashes with assert(field != __null) failed: missing field

[Aleksey Shipilev]

Original fix:
   https://bugs.openjdk.java.net/browse/JDK-8250825
   https://hg.openjdk.java.net/jdk/jdk/rev/bd4d4ab3c2c1

11u does not have is_reference_type helper, and so the patch does not apply cleanly.

11u variant:
   https://cr.openjdk.java.net/~shade/8250825/webrev.11u.01/

Testing: new test (fails without the fix, passes with it); tier{1,2}

-- 
Thanks,
-Aleksey