Support text export of fusion subgraphs

[hgaspar]

Add a command line argument to the migraphx driver (for example export_mlir), which, when set, will perform the following:
For each mlir fusion (module) that migraphx decides to perform, the corresponding IR will be exported in a text file (.mlir); The filename will ideally be reflective of what the fusion is about (e.g. via concat of the names or the operators, appended into the model name).

Those exported files will be used as follows:
Test scripts in rocMLIR will import each such file, compile the code to create a single code object, verify that this succeeds, and if it does, "verify" the correctness, and benchmark the execution time; and, finally, write all that info in a structured way, so that results from different runs can be compared.