[mlir][arith] Extend the `floordivsi` converter

This patch extends the `createConst` method so that it can generate
constant vectors (it can already generate scalars). This change is
required to be able to apply the converter for `arith.floordivsi`
(i.e. `FloorDivSIOpConverter`) to vectors.

While `arith.floordivsi` is my main motivation for this change, this
patch should also allow other Arith ops to be converted in vector cases.
In my example, the Linalg vectorizer updates `arith.floordivsi` to
operate on vectors and hence the need for this change.

Differential Revision: https://reviews.llvm.org/D146741