[VPlan] Guard VPPartialReductionRecipe::computeCost() against ilist sentinel(#148389)

[shashforge]

VPValue::getDefiningRecipe() can return the ilist sentinel for live-in
values. Dereferencing that sentinel trips the “dyn_cast on a non-existent
value” assertion (PR #148389).

• Collapse sentinel → nullptr with a small helper.
• Replace plain isa/dyn_cast with isa_and_present / dyn_cast_if_present.
• Keep old behaviour for non-sentinel recipes, so the chosen VF doesn’t change.

Added lit test Transforms/LoopVectorize/pr148389-sentinel.ll.

Fixes: #148389

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff HEAD~1 HEAD --extensions cpp -- llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

View the diff from clang-format here.

diff --git a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
index fc918c669..5f4fc4c32 100644
--- a/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
+++ b/llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp
@@ -301,11 +301,11 @@ VPPartialReductionRecipe::computeCost(ElementCount VF,
   // -----------------------------------------------------------------------
   auto safe = [](VPRecipeBase *R) -> VPRecipeBase * {
     return isa_and_present<VPRecipeBase>(R) ? R : nullptr;
-   };
+  };
 
   std::optional<unsigned> Opcode;
-  VPValue       *Op  = getOperand(0);
-  VPRecipeBase  *OpR = safe(Op->getDefiningRecipe());
+  VPValue *Op = getOperand(0);
+  VPRecipeBase *OpR = safe(Op->getDefiningRecipe());
 
   // If the partial reduction is predicated, a select will be operand 0
   using namespace llvm::VPlanPatternMatch;