[MLIR] PresburgerSet emptiness check: remove assertions that there are no symbols

[arjunp]

Summary

Symbols are now supported in the integer emptiness check. Remove some outdated assertions checking that there are no symbols.

mlir/lib/Analysis/PresburgerSet.cpp

bool PresburgerSet::isEqual(const PresburgerSet &set) const {
   assertDimensionsCompatible(set, *this);
   return this->subtract(set).isIntegerEmpty() &&
          set.subtract(*this).isIntegerEmpty();
 }
 
 /// Return true if all the sets in the union are known to be integer empty,
 /// false otherwise.
 bool PresburgerSet::isIntegerEmpty() const {
-  assert(nSym == 0 && "isIntegerEmpty is intended for non-symbolic sets");
   // The set is empty iff all of the disjuncts are empty.
   for (const FlatAffineConstraints &fac : flatAffineConstraints) {
     if (!fac.isIntegerEmpty())
       return false;
   }
   return true;
 }
 
 bool PresburgerSet::findIntegerSample(SmallVectorImpl<int64_t> &sample) {
-  assert(nSym == 0 && "findIntegerSample is intended for non-symbolic sets");
   // A sample exists iff any of the disjuncts contains a sample.
   for (const FlatAffineConstraints &fac : flatAffineConstraints) {
     if (Optional<SmallVector<int64_t, 8>> opt = fac.findIntegerSample()) {
       sample = std::move(*opt);
       return true;
     }
   }
   return false;