[BasicTTI] Only split vectors with even element counts in getCastInstrCost (#166528)

Fixes #166320

Author: ahmedshakill

llvm/include/llvm/CodeGen/BasicTTIImpl.h

@@ -1331,8 +1331,8 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
       bool SplitDst =
           TLI->getTypeAction(Dst->getContext(), TLI->getValueType(DL, Dst)) ==
           TargetLowering::TypeSplitVector;
-      if ((SplitSrc || SplitDst) && SrcVTy->getElementCount().isVector() &&
-          DstVTy->getElementCount().isVector()) {
+      if ((SplitSrc || SplitDst) && SrcVTy->getElementCount().isKnownEven() &&
+          DstVTy->getElementCount().isKnownEven()) {
         Type *SplitDstTy = VectorType::getHalfElementsVectorType(DstVTy);
         Type *SplitSrcTy = VectorType::getHalfElementsVectorType(SrcVTy);
         const T *TTI = thisT();

llvm/test/Transforms/VectorCombine/AArch64/sve-interleave-splat.ll

@@ -0,0 +1,11 @@
+; RUN: opt -passes=vector-combine %s -S -o - | FileCheck %s
+
+target triple = "aarch64-unknown-linux-gnu"
+
+define <vscale x 4 x i16> @interleave2_same_const_splat_nxv4i16() {
+;CHECK-LABEL: @interleave2_same_const_splat_nxv4i16(
+;CHECK: call <vscale x 4 x i16> @llvm.vector.interleave2
+;CHECK: ret <vscale x 4 x i16> %retval
+  %retval = call <vscale x 4 x i16> @llvm.vector.interleave2.nxv4i16(<vscale x 2 x i16> splat(i16 3), <vscale x 2 x i16> splat(i16 3))
+  ret <vscale x 4 x i16> %retval
+}