nodes to indicate when ha16/lo16 modifiers should be used.  This lets
us pass PowerPC/indirectbr.ll.

The one annoying thing about this patch is that the MCSymbolExpr isn't
expressive enough to represent ha16(label1-label2) which we need on
PowerPC.  I have a terrible hack in the meantime, but this will have
to be revisited at some point.

Last major conversion item left is global variable references.

llvm-svn: 119105