Many targets build placeholder nodes for special operands, e.g.
GlobalBaseReg on X86 and PPC for the PIC base.  There's no
sensible way to associate debug info with these.  I've left
them built with getNode calls with explicit DebugLoc::getUnknownLoc operands. 
I'm not too happy about this but don't see a good improvement;
I considered adding a getPseudoOperand or something, but it
seems to me that'll just make it harder to read.

llvm-svn: 63992