[mlir][linalg] Unify generic vectorization interface.

It breaks the logic of maskedVectorize (on tensor.pad ops) into
precondition checks and vectorization implementation; unifies the
interface.

The revision also rename`s vectorizeLinalgOpPrecondition` to
`vectorizeOpPrecondition` because we can vectorize ops other
than LinalgOps.

Reviewed By: dcaballe

Differential Revision: https://reviews.llvm.org/D150495