There is currently no lowering out of `ml_program` in the LLVM
repository. This change adds a lowering to `memref` so that it can be
lowered all the way to LLVM. This lowering was taken from the [reference
backend in
torch-mlir](f416953600
).
I had tried implementing the `BufferizableOpInterface` for `ml_program`
instead of adding a new pass but that did not work because
`OneShotBufferize` does not visit module-level ops like
`ml_program.global`.
