i965/fs: Debug the optimization passes by dumping instr to file.

i965/fs: Debug the optimization passes by dumping instr to file.

With INTEL_DEBUG=optimizer, write the output of dump_instructions() to a
file each time an optimization pass makes progress. This lets you easily
diff successive files to see what an optimization pass did.

Example filenames written when running glxgears:
   fs8-0000-00-start
   fs8-0000-01-04-opt_copy_propagate
   fs8-0000-01-06-dead_code_eliminate
   fs8-0000-01-12-compute_to_mrf
   fs8-0000-02-06-dead_code_eliminate
        |   |  |   |
        |   |  |   `-- optimization pass name
        |   |  |
        |   |  `-- optimization pass number in the loop
        |   |
        |   `-- optimization loop interation
        |
        `-- shader program number

Note that with INTEL_DEBUG=optimizer, we disable compact_virtual_grfs,
so that we can diff instruction lists across loop interations without
the register numbers being changes.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>