Currently in `combineVectorShuffle()`, we update the shuffle mask if either

input vector comes from a scalar_to_vector, and we keep the respective input

vectors in its permuted form by producing `PPCISD::SCALAR_TO_VECTOR_PERMUTED`.

However, it is possible that we end up in a situation where both input vectors

to the vector_shuffle are scalar_to_vector, and are different vector types.

In situations like this, the shuffle mask is updated incorrectly as the current

code assumes both scalar_to_vector inputs are the same vector type.

This patch skips the combines for vector_shuffle if both input vectors are

scalar_to_vector, and if they are of different vector types. A follow up patch

will focus on fixing this issue afterwards, in order to correctly update the

shuffle mask.