IndependentBlocks: Do not assert for PHI nodes outside of scops

There does not seem to be a reason that we can not support PHI nodes outside of
the scop that reference values within the SCoP. Or at least, the attached test
case seems to do the right thing. We remove the assert for now.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@200427 91177308-0d34-0410-b5e6-96231b3b80d8