[mlir] Use getAffineExprFromFlatForm for simplifying flatten cases.

[hanchung]

Summary

There is a bug in getSemiAffineExprFromFlatForm. The
getAffineExprFromFlatForm method can handle the case, but the other
fails. Switch to working method to unblock developments of downstream
projects.

Minimized test case:

#map2 = affine_map<()[s0] -> (((s0 floordiv 2) * s0) + (s0 mod 2))>
module {
  func.func @dynamic_unpack_simple_dispatch_0(%arg3: index) {
    %3 = affine.apply #map2()[%arg3] {__d__}
    vector.print %3 : index
    return
  }
}

See https://github.com/llvm/llvm-project/issues/58419 for more details.

[hanchung]

There are three failed tests. I can fix the tests if we can land the change.

Failed Tests (3):
  MLIR :: Dialect/Affine/simplify-structures.mlir
  MLIR :: Dialect/Tensor/fold-consecutive-insert-extract-slice.mlir
  MLIR :: Examples/standalone/test.toy

[arnab-oss]

This patch fixes the repeating index issue.

mlir/lib/IR/AffineExpr.cpp

AffineExpr mlir::simplifyAffineExpr(AffineExpr expr, unsigned numDims,
   SimpleAffineExprFlattener flattener(numDims, numSymbols);
   flattener.walkPostOrder(expr);
   ArrayRef<int64_t> flattenedExpr = flattener.operandExprStack.back();
   if (!expr.isPureAffine() &&
       expr == getAffineExprFromFlatForm(flattenedExpr, numDims, numSymbols,
                                         flattener.localExprs,
                                         expr.getContext()))
     return expr;
+  // TODO: Use getSemiAffineExprFromFlatForm method after the bug is being
+  // fixed.
   AffineExpr simplifiedExpr =
-      expr.isPureAffine()
-          ? getAffineExprFromFlatForm(flattenedExpr, numDims, numSymbols,
-                                      flattener.localExprs, expr.getContext())
-          : getSemiAffineExprFromFlatForm(flattenedExpr, numDims, numSymbols,
-                                          flattener.localExprs,
-                                          expr.getContext());
+      getAffineExprFromFlatForm(flattenedExpr, numDims, numSymbols,
+                                flattener.localExprs, expr.getContext());
 
   flattener.operandExprStack.pop_back();
   assert(flattener.operandExprStack.empty());
   return simplifiedExpr;
 }