[mlir]Add a check to ensure bailing out when reducing to a scalar (llvm#129694)

Fixes issue llvm#64075
Referencing this comment for more detailed view ->
llvm#64075 (comment)

**Minimal example crashing :** 
```
func.func @multi_reduction(%0: vector<4x2xf32>, %acc1: f32) -> f32 {
  %2 = vector.multi_reduction <add>, %0, %acc1 [0, 1] : vector<4x2xf32> to f32
  return %2 : f32
}
```

Author: Prakhar-Dixit

mlir/lib/Dialect/Vector/Transforms/VectorUnroll.cpp

@@ -355,6 +355,11 @@ struct UnrollMultiReductionPattern
 
   LogicalResult matchAndRewrite(vector::MultiDimReductionOp reductionOp,
                                 PatternRewriter &rewriter) const override {
+    auto resultType = reductionOp->getResult(0).getType();
+    if (resultType.isIntOrFloat()) {
+      return rewriter.notifyMatchFailure(reductionOp,
+                                         "Unrolling scalars is not supported");
+    }
     std::optional<SmallVector<int64_t>> targetShape =
         getTargetShape(options, reductionOp);
     if (!targetShape)

mlir/test/Dialect/Vector/vector-unroll-options.mlir

@@ -222,6 +222,15 @@ func.func @vector_multi_reduction(%v : vector<4x6xf32>, %acc: vector<4xf32>) ->
 //       CHECK:   %[[V2:.*]] = vector.insert_strided_slice %[[R5]], %[[V1]] {offsets = [2], strides = [1]} : vector<2xf32> into vector<4xf32>
 //       CHECK:   return %[[V2]] : vector<4xf32>
 
+// This is a negative test case to ensure that further unrolling is not performed. Since the vector.multi_reduction
+// operation has already been unrolled, attempting additional unrolling should not be allowed.
+func.func @negative_vector_multi_reduction(%v: vector<4x2xf32>, %acc: f32) -> f32 {
+  %0 = vector.multi_reduction #vector.kind<add>, %v, %acc [0, 1] : vector<4x2xf32> to f32
+  return %0 : f32
+}
+// CHECK-LABEL: func @negative_vector_multi_reduction
+//  CHECK-NEXT:   %[[R0:.*]] = vector.multi_reduction <add>, %{{.*}}, %{{.*}} [0, 1] : vector<4x2xf32> to f32
+//  CHECK-NEXT:   return %[[R0]] : f32
 
 func.func @vector_reduction(%v : vector<8xf32>) -> f32 {
   %0 = vector.reduction <add>, %v : vector<8xf32> into f32