[llvm] PreISelIntrinsicLowering: Lower llvm.exp to a loop if scalable vec arg (PR #117568)

[Matt Arsenault via llvm-commits]

================
@@ -32,26 +28,34 @@ bool llvm::lowerUnaryMathIntrinsicWithScalableVecArgAsLoop(Module &M,
   BasicBlock *LoopBB = BasicBlock::Create(Ctx, "", ParentFunc, PostLoopBB);
   PreLoopBB->getTerminator()->setSuccessor(0, LoopBB);
 
-  // loop preheader
+  // Loop preheader
   IRBuilder<> PreLoopBuilder(PreLoopBB->getTerminator());
-  Value *VScale = PreLoopBuilder.CreateVScale(
-      ConstantInt::get(PreLoopBuilder.getInt64Ty(), 1));
-  Value *N = ConstantInt::get(PreLoopBuilder.getInt64Ty(),
-                              ScalableTy->getMinNumElements());
-  Value *LoopEnd = PreLoopBuilder.CreateMul(VScale, N);
+  Value *LoopEnd = nullptr;
+  if (VecTy->isScalableTy()) {
+    ScalableVectorType *ScalableVecTy = cast<ScalableVectorType>(VecTy);
----------------
arsenm wrote:

dyn_cast instead of cast + isScalableTy

https://github.com/llvm/llvm-project/pull/117568