[CostModel] improveShuffleKindFromMask - recognise a SK_PermuteSingleSrc incorrectly tagged as SK_PermuteTwoSrc (#145352)

If a SK_PermuteTwoSrc shuffle kind's mask only references the first
operand, then treat this as SK_PermuteSingleSrc

Part of #145335

Author: RKSimon

llvm/include/llvm/CodeGen/BasicTTIImpl.h

@@ -1122,6 +1122,9 @@ class BasicTTIImplBase : public TargetTransformInfoImplCRTPBase<T> {
       break;
     }
     case TTI::SK_PermuteTwoSrc: {
+      if (all_of(Mask, [NumSrcElts](int M) { return M < NumSrcElts; }))
+        return improveShuffleKindFromMask(TTI::SK_PermuteSingleSrc, Mask, SrcTy,
+                                          Index, SubTy);
       int NumSubElts;
       if (Mask.size() > 2 && ShuffleVectorInst::isInsertSubvectorMask(
                                  Mask, NumSrcElts, NumSubElts, Index)) {