[9] RFR (XS): 8060483: NPE with explicitCastArguments unboxing null

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8060483/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8060483

Recent changes broke MHs.explicitCastArguments for null values when 
unboxing taking place.

The fix corrects the equivalence check for MHs.eCA & MH.asType() in 
MT.explicitCastEquivalentToAsType().

Also, will file a followup enhancement request to improve test coverage 
for MHs.eCA method.

Testing: regression test, jck (api/java_lang/invoke), jdk/java/lang/invoke

Thanks!

Best regards,
Vladimir Ivanov