[Vectorize] Update comment of getSubdividedVectorType (#107632)

The original comment here is wrong, as demonstrated by the included
test.

Update the comment to reflect what getSubdividedVectorType actually
does.

Author: Sterling-Augustine

llvm/include/llvm/IR/DerivedTypes.h

@@ -492,9 +492,9 @@ class VectorType : public Type {
     return VectorType::get(EltTy, VTy->getElementCount());
   }
 
-  // This static method returns a VectorType with a smaller number of elements
-  // of a larger type than the input element type. For example, a <16 x i8>
-  // subdivided twice would return <4 x i32>
+  // This static method returns a VectorType with a larger number of elements
+  // of a smaller type than the input element type. For example, a <4 x i64>
+  // subdivided twice would return <16 x i16>
   static VectorType *getSubdividedVectorType(VectorType *VTy, int NumSubdivs) {
     for (int i = 0; i < NumSubdivs; ++i) {
       VTy = VectorType::getDoubleElementsVectorType(VTy);

llvm/unittests/IR/VectorTypesTest.cpp

@@ -125,6 +125,10 @@ TEST(VectorTypesTest, FixedLength) {
   EltCnt = V8Int64Ty->getElementCount();
   EXPECT_EQ(EltCnt.getKnownMinValue(), 8U);
   ASSERT_FALSE(EltCnt.isScalable());
+
+  auto *SubTy = VectorType::getSubdividedVectorType(V4Int64Ty, 2);
+  EXPECT_EQ(SubTy->getElementCount(), ElementCount::getFixed(16));
+  EXPECT_TRUE(SubTy->getElementType()->isIntegerTy(16));
 }
 
 TEST(VectorTypesTest, Scalable) {