[llvm] [LV][NFC] Change undef to poison in most tests (PR #163525)

[Florian Hahn via llvm-commits]

================
@@ -40,7 +40,7 @@ for.body:                                         ; preds = %for.body, %entry
   br i1 %exitcond, label %for.end, label %for.body
 
 for.end:                                          ; preds = %for.body
-  ret i32 undef
+  ret i32 poison
----------------
fhahn wrote:

better to use`ret void` instead

https://github.com/llvm/llvm-project/pull/163525