Fix a crash in block placement due to an inner loop that happened to be

Fix a crash in block placement due to an inner loop that happened to be
reversed in the function's original ordering, and we happened to
encounter it while handling an outer unnatural CFG structure.

Thanks to the test case reduced from GCC's source by Benjamin Kramer.
This may also fix a crasher in gzip that Duncan reduced for me, but
I haven't yet gotten to testing that one.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@145094 91177308-0d34-0410-b5e6-96231b3b80d8