Fix (at least) quadratic worst-case complexity in DominanceFrontier::splitBlock:

don't visit all blocks in the function, and don't iterate over the split blocks'
predecessor lists for each block visited.

Also, remove the special-case test for the entry block. Splitting the entry
block isn't common enough to make this worthwhile.

This fixes a major compile-time bottleneck which is exposed now that
LoopSimplify isn't being redundantly run both before and after
DominanceFrontier.

llvm-svn: 109408