RFR(S) 8032656: Tag the MachSpillCopies with purpose information

[Niclas Adlertz]

Hi all,

When debugging or visualizing spills/splits/moves in C2 it's hard to 
know what caused an insertion of a certain MachSpillCopy. I've made this 
easier by creating subclasses of the MachSpillCopyNode.

webrev: http://cr.openjdk.java.net/~adlertz/JDK-8032656/webrev00/
bug: https://bugs.openjdk.java.net/browse/JDK-8032656

Kind Regards,
Niclas Adlertz