Remove an over-eager assert that was firing on one of the ARM regression

Remove an over-eager assert that was firing on one of the ARM regression
tests when I forcibly enabled block placement.

It is apparantly possible for an unanalyzable block to fallthrough to
a non-loop block. I don't actually beleive this is correct, I believe
that 'canFallThrough' is returning true needlessly for the code
construct, and I've left a bit of a FIXME on the verification code to
try to track down why this is coming up.

Anyways, removing the assert doesn't degrade the correctness of the algorithm.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144532 91177308-0d34-0410-b5e6-96231b3b80d8