RFR(XS): 8155244 - JVMCI: MemoryAccessProvider.readUnsafeConstant javadoc should be updated for null JavaKind case

[Dmitrij Pochepko]

Hi,

please review changes for 8155244 - JVMCI: 
MemoryAccessProvider.readUnsafeConstant javadoc should be updated for 
null JavaKind case

MemoryAccessProvider.readUnsafeConstant method implementation throws 
NullPointerException in case JavaKind is null. Such behavior wasn't 
specified in javadoc.
So, a small fix contains logic to throw IllegalArgumentException and 
respective javadoc update.

CR: https://bugs.openjdk.java.net/browse/JDK-8155244
webrev: http://cr.openjdk.java.net/~dpochepk/8155244/webrev.01/

I've tested fix on linux_x64 via jprt.

Thanks,
Dmitrij