Improve ext(mul(ext, ext)) comment

Author: SamTebbs33

llvm/lib/Transforms/Vectorize/VPlanTransforms.cpp

@@ -3713,9 +3713,11 @@ tryToMatchAndCreateMulAccumulateReduction(VPReductionRecipe *Red,
     // -> reduce.add(ext(mul(ext, ext(const))))
     ExtendAndReplaceConstantOp(Ext0, Ext1, B, Mul);
 
-    // Match reduce.add(ext(mul(ext(A), ext(B))))
-    // All extend recipes must have same opcode or A == B
-    // which can be transformed to reduce.add(zext(mul(sext(A), sext(B)))).
+    // reduce.add(ext(mul(ext(A), ext(B))))
+    // -> reduce.add(mul(wider_ext(A), wider_ext(B)))
+    // The inner extends must either have the same opcode as the outer extend or
+    // be the same, in which case the multiply can never result in a negative
+    // value and the outer extend opcode doesn't matter
     if (Ext0 && Ext1 &&
         (Ext->getOpcode() == Ext0->getOpcode() || Ext0 == Ext1) &&
         Ext0->getOpcode() == Ext1->getOpcode() &&