Don't attempt to split subloops out of a loop with a huge number of backedges.

Not only will this take huge amounts of compile time, the resultant loop nests
won't be useful for optimization.  This reduces loopsimplify time on
Transforms/LoopSimplify/2006-08-11-LoopSimplifyLongTime.ll from ~32s to ~0.4s
with a debug build of llvm on a 2.7Ghz G5.

llvm-svn: 29647