[GISel][CombinerHelper] Combine shuffle_vector scalar to build_vector

Teach the combiner helper how to replace shuffle_vector of scalars
into build_vector.
I am not particularly happy about having to add this combine, but we
currently get those from <1 x iN> from the IR.

Bonus: This fixes an assert in the shuffle_vector combines since before
this patch, we were expecting vector types.