[PlaceSafepoints] Followup to commit L237172

Responding to review feedback from http://reviews.llvm.org/D9585

1) Remove a variable shadow by converting the outer loop to a range for loop.  We never really used the 'i' variable which was being shadowed.
2) Reduce DominatorTree recalculations by passing the DT to SplitEdge.

llvm-svn: 237212