[mlir] [VectorOps] Merge OUTER/AXPY vector.contract lowering into single case

We temporarily had separate OUTER lowering (for matmat flavors) and
AXPY lowering (for matvec flavors). With the new generalized
"vector.outerproduct" semantics, these cases can be merged into
a single lowering method. This refactoring will simplify future
decisions on cost models and lowering heuristics.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D83585