This patch fixes the combines for vector_shuffles when either or both of its
left and right hand side inputs are scalar_to_vector nodes.
Previously, when both left and right side inputs are scalar_to_vector nodes,
the current combine could not handle this situation, as the shuffle mask was
updated incorrectly. https://reviews.llvm.org/D127818 was a temporary solution
to this issue. Now, not only does this patch aim to resolve the previous issue the
of incorrect shuffle mask adjustments respectively, it also updates any test cases
that are affected by this change.