[SystemZ] Rename mapping table fields

Rename Function->DispKey and PairType->DispSize.  I'd originally used
"Function" because I thought it might be useful for other InstMappings.
However, it turns out that having two very similar instructions with the
same Function makes it pretty useless for anything other than the displacement
size key.  Other InstMappings will want the key to be defined for only one
instruction in the pair.

No behavioural change intended.

llvm-svn: 185526