Fix ppc store update instructions

[SeekyCt]

Currently, indexed store update instructions give an error along the lines of

        // Error: Expected instruction argument to be of the form offset($register), but found $r3
        // At instruction: stwux $r9, $r3, $r5

due to the call of a.memory_ref in the eval_fn (which is only valid for the non-indexed versions).

This PR fixes this by splitting out the eval_fn into separate ones for the indexed and non-indexed versions like load update instructions do

One thing that's worth noting is that none of the current tests contain any of these instructions, and #187 doesn't actually list them; Super Paper Mario has some functions using the instructions if that helps (possibly means that only later CW versions actually emitted them)

[simonlindholm]

Makes sense. Thanks! The note about Super Paper Mario is useful, sounds like we should use that for project tests if m2c works picks up more again in the future.