[llvm] [LV] Use getFixedValue instead of getKnownMinValue when appropriate (PR #143526)

[Luke Lau via llvm-commits]

================
@@ -5342,6 +5342,10 @@ LoopVectorizationCostModel::getUniformMemOpCost(Instruction *I,
   StoreInst *SI = cast<StoreInst>(I);
 
   bool IsLoopInvariantStoreValue = Legal->isInvariant(SI->getValueOperand());
+  // TODO: We have tests that request the cost of extracting element
+  // VF.getKnownMinValue() - 1 from a scalable vector. This is actually
+  // meaningless, given what we actually want is the last lane and is likely
+  // to be more expensive.
----------------
lukel97 wrote:

Maybe we could change the Index argument of `getVectorInstrCost` to be an ElementCount?

https://github.com/llvm/llvm-project/pull/143526