[m-dev.] diff: simplify.m bug fix

[Simon TAYLOR]

> compiler/simplify.m:
> 	Fix a bug: the code was passing around two different copies
> 	of ModuleInfo, one passed directly, one stored in the det_info
> 	in the simplify_info, and these were not being kept in sync.
> 	That caused the final ModuleInfo to contain the wrong merge_inst
> 	table, which led to map__lookup failures.

That looks fine, thanks Fergus.