Support text export of fusion subgraphs

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.

Start by locating the migraphx driver and the code that decides which MLIR fusions to perform. Review how rocMLIR test scripts consume generated .mlir files and what structured results they expect. Done means an export argument produces identifiable text files for each fusion and those files can support compilation, correctness checks, benchmarking, and result comparison.