[PowerPC] Fix combineVectorShuffle regression after D77448

Commit 1fed1316 assumed that NewShuffle (shuffle vector
canonicalization result) will always be ShuffleVectorSDNode, which may
be false (it may be a BITCAST node):

```
...
t12: v4i32 = scalar_to_vector t2
t15: v16i8 = bitcast t12  # LHS
t17: v16i8 = vector_shuffle<u,u,u,u,u,u,u,u,0,1,2,3,u,u,u,u> t15, undef:v16i8  # SVN
```

Reviewed By: #powerpc, nemanjai

Differential Revision: https://reviews.llvm.org/D83617