Swift SVN r12226
This complements the DominanceInfo class. Swift SVN r10736
Swift SVN r6421
type, though; I had to define a WriteAsOperand function and add a front() method to SILFunction to get this to work. Rip out my dominators implementation and replace it with LLVM's. I'd forgotten that LLVM's was actually generic. Swift SVN r4717
Kennedy (2001). This hasn't been tested yet. Swift SVN r4712