Teach machine block placement to cope with unnatural loops. These don't

Teach machine block placement to cope with unnatural loops. These don't
get loop info structures associated with them, and so we need some way
to make forward progress selecting and placing basic blocks. The
technique used here is pretty brutal -- it just scans the list of blocks
looking for the first unplaced candidate. It keeps placing blocks like
this until the CFG becomes tractable.

The cost is somewhat unfortunate, it requires allocating a vector of all
basic block pointers eagerly. I have some ideas about how to simplify
and optimize this, but I'm trying to get the logic correct first.

Thanks to Benjamin Kramer for the reduced test case out of GCC. Sadly
there are other bugs that GCC is tickling that I'm reducing and working
on now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144516 91177308-0d34-0410-b5e6-96231b3b80d8