[m-dev.] diff: bug fix for recompute_instmap_delta on switches

[Simon TAYLOR]

Fergus wrote:
> compiler/mode_util.m:
> 	Fix bugs in recompute_instmap_delta_cases.
> 	The old code was passing the wrong instmap to
> 	instmap_delta_bind_var and in the recursive call.
> 	It was passing the instmap _after_ the implicit
> 	functor test unification at the start of the case.
> 	For example, for a switch like this:

That looks fine, thanks Fergus.