swift234

two conflated changes:
1) Sprinkle some ilist_traits magic in to have the ilist completely manage
   the parent pointers of instructions in a block.  As an instruction is 
   added or removed from the list, the parent pointers are automatically
   kept up to date.
2) Switch DeclRefInst over to CFGBuilder API for construction instead of
   'new-style' api.


Swift SVN r2764