Add missing lowering to CFG in mlir-cpu-runner + related cleanup

- the list of passes run by mlir-cpu-runner included -lower-affine and
  -lower-to-llvm but was missing -lower-to-cfg (because -lower-affine at
  some point used to lower straight to CFG); add -lower-to-cfg in
  between. IR with affine ops can now be run by mlir-cpu-runner.

- update -lower-to-cfg to be consistent with other passes (create*Pass methods
  were changed to return unique ptrs, but -lower-to-cfg appears to have been
  missed).

- mlir-cpu-runner was unable to parse custom form of affine op's - fix
  link options

- drop unnecessary run options from test/mlir-cpu-runner/simple.mlir
  (none of the test cases had loops)

- -convert-to-llvmir was changed to -lower-to-llvm at some point, but the
  create pass method name wasn't updated (this pass converts/lowers to LLVM
  dialect as opposed to LLVM IR). Fix this.

(If we prefer "convert", the cmd-line options could be changed to
"-convert-to-llvm/cfg" then.)

Signed-off-by: Uday Bondhugula <uday@polymagelabs.com>

Closes tensorflow/mlir#115

PiperOrigin-RevId: 266666909