Make a somewhat subtle change in the logic of block placement. Sometimes

Make a somewhat subtle change in the logic of block placement. Sometimes
the loop header has a non-loop predecessor which has been pre-fused into
its chain due to unanalyzable branches. In this case, rotating the
header into the body of the loop in order to place a loop exit at the
bottom of the loop is a Very Bad Idea as it makes the loop
non-contiguous.

I'm working on a good test case for this, but it's a bit annoynig to
craft. I should get one shortly, but I'm submitting this now so I can
begin the (lengthy) performance analysis process. An initial run of LNT
looks really, really good, but there is too much noise there for me to
trust it much.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@154395 91177308-0d34-0410-b5e6-96231b3b80d8