[ISLTools/ZoneAlgo] Make distributeDomain and filterKnownValInst isl_error_quota proof.

distributeDomain() and filterKnownValInst() are used in a scop
of ForwardOpTree that limits the number of isl operations.
Therefore some isl functions may return null after any operation.

Remove assertion that assume non-null results and handle
isl_*_foreach returning isl::stat::error.

I hope this fixes the crash of the asop buildbot at ihevc_recon.c.

llvm-svn: 310461