[9] RFR (S) 8057656: Improve MethodType.isCastableTo() & MethodType.isConvertibleTo() checks

[Vladimir Ivanov]

http://cr.openjdk.java.net/~vlivanov/8057656/webrev.00/
https://bugs.openjdk.java.net/browse/JDK-8057656

There are some corner cases which MT.isCastableTo() & 
MT.isConvertibleTo() don't treat right (e.g. int->String converstion of 
return type in MT.isCastableTo()).

Testing: jdk/java/lang/invoke, jdk/java/util/streams, nashorn, octane w/ 
"-ea -esa" and COMPILE_THRESHOLD={0,30}.

Reviewed-by: vlivanov, ?
Contributed-by: john.r.rose at oracle.com

Thanks!


Best regards,
Vladimir Ivanov