[mlir][vector] NFC: Improve vector type accessor methods

Plain `getVectorType()` can be quite confusing and error-prone
given that, well, vector ops always work on vector types, and
it can commonly involve both source and result vectors. So this
commit makes various such accessor methods to be explicit w.r.t.
source or result vectors.

Reviewed By: ThomasRaoux

Differential Revision: https://reviews.llvm.org/D144159