[ZoneAlgo] Make ZoneAlgorithm::isNormalized out-of-quota safe.

The aosp-O3-polly-before-vectorizer-unprofitable buildbot currently
fails in ZoneAlgorithm::isNormalized, presumably because an
out-of-quota happens in that function.

Modify ZoneAlgorithm::isNormalized to return an isl::boolean such
it can report an error.

In the failing case, it was called in an assertion in ForwardOpTree.
Allow to pass the assertion in an out-of-quota event, a condition that
is later checked before forwarding an operand tree.

llvm-svn: 333709