[llvm] [loop-vectorize] Fix crash when using types that aren't known scale factors (PR #136680)

[Florian Hahn via llvm-commits]

================
@@ -1030,6 +1030,28 @@ for.body:                                         ; preds = %for.body.preheader,
   br i1 %exitcond.not, label %for.cond.cleanup, label %for.body, !loop !1
 }
 
+
+define void @chained_partial_reduce_not_known_factor(i32 %a, i32 %b, i32 %N) #0 {
+entry:
+  br label %for.body
+
+for.cond.cleanup:
+  %B.0.lcssa = phi i40 [ %2, %for.body ]
+  ret void
+
+for.body:
+  %B.017 = phi i40 [ 0, %entry ], [ %2, %for.body ]
+  %i.016 = phi i16 [ 0, %entry ], [ %add21, %for.body ]
----------------
fhahn wrote:

```suggestion
  %iv = phi i16 [ 0, %entry ], [ %iv.next, %for.body ]
```

https://github.com/llvm/llvm-project/pull/136680