[GlobalISel] Handle <1 x T> vector return types properly.
After support for dealing with types that need to be extended in some way was
added in r358032 we didn't correctly handle <1 x T> return types. These types
don't have a GISel direct representation, instead we just see them as scalars.
When we need to pad them into <2 x T> types however we need to use a
G_BUILD_VECTOR instead of trying to do a G_CONCAT_VECTOR.
This fixes PR41738.