[PM/LoopUnswitch] Remove another over-aggressive assert.

This code path can very clearly be called in a context where we have
baselined all the cloned blocks to a particular loop and are trying to
handle nested subloops. There is no harm in this, so just relax the
assert. I've added a test case that will make sure we actually exercise
this code path.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@330680 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed