[llvm] [LV] Use frozen start value for FindLastIV if needed. (PR #132691)

[David Sherwood via llvm-commits]

================
@@ -10377,6 +10380,36 @@ static void preparePlanForMainVectorLoop(VPlan &MainPlan, VPlan &EpiPlan) {
       VPInstruction::ResumePhi,
       {VectorTC, MainPlan.getCanonicalIV()->getStartValue()}, {},
       "vec.epilog.resume.val");
+
+  // When vectorizing the epilogue, FindLastIV reductions can introduce multiple
+  // uses of undef/poison. If the reduction start value is not guaranteed to be
----------------
david-arm wrote:

I know this makes it even wordier, but shouldn't this be `If the reduction start value is not guaranteed to **not** be undef or poison`? In other words, if the start value could be either undef or poison we need to freeze it?

https://github.com/llvm/llvm-project/pull/132691