[Matrix] Add asserts for mismatched element types.

This patch clarifies the failing point of having input or output vectors
of differing types. Before, lowering would fail elsewhere (e.g. in
`fmul` creation) which may have been not immediately clear.

As a side effect, the `getElementType` and `getVectoryTy` functions
required the `const` qualifier to be added.

Reviewers: fhahn

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D84374