[mlir][Analysis] Fix assertion in FlatLinearConstraints

[Groverkss]

LGTM. Thanks!

mlir/lib/Analysis/FlatLinearValueConstraints.cpp

 /// variables (dimensional and symbolic variables). Local variables are
 /// themselves explicitly computed as affine functions of other variables in
 /// this process if needed.
 void FlatLinearConstraints::getSliceBounds(unsigned offset, unsigned num,
                                            MLIRContext *context,
                                            SmallVectorImpl<AffineMap> *lbMaps,
                                            SmallVectorImpl<AffineMap> *ubMaps,
                                            bool closedUB) {
-  assert(num < getNumDimVars() && "invalid range");
+  assert(offset + num <= getNumDimVars() && "invalid range");
 
   // Basic simplification.
   normalizeConstraintsByGCD();
 
   LLVM_DEBUG(llvm::dbgs() << "getSliceBounds for first " << num
                           << " variables\n");
   LLVM_DEBUG(dump());