[Hexagon] Don't use garbage mask in HvxSelector::shuffp2

The function shuffp2 was breaking up a wide shuffle into a pair of
narrower ones, except that the narrower shuffle masks were actually
uninitialized.

llvm-svn: 324243

llvm/lib/Target/Hexagon/HexagonISelDAGToDAGHVX.cpp

@@ -1271,6 +1271,8 @@ OpRef HvxSelector::shuffp2(ShuffleMask SM, OpRef Va, OpRef Vb,
     return shuffp1(ShuffleMask(PackedMask), P, Results);
 
   SmallVector<int,256> MaskL(VecLen), MaskR(VecLen);
+  splitMask(SM.Mask, MaskL, MaskR);
+
   OpRef L = shuffp1(ShuffleMask(MaskL), Va, Results);
   OpRef R = shuffp1(ShuffleMask(MaskR), Vb, Results);
   if (!L.isValid() || !R.isValid())