[mlir][vector] Add scalable vectors support to OuterProductOp

This will probably be the first in a series of patches that tries to
enable code generation for ARM SME (extension of SVE).

Since SME's core operation is the outer product instruction, I figured
that it would probably be a good idea to enable the outer product
operation to properly accept and generate scalable vectors.

Reviewed By: dcaballe

Differential Revision: https://reviews.llvm.org/D138718