There were two related bugs here

Trac #9426
   We must increment the ic_mod_index field of the InteractiveContext
   if we have new instances, because we maek DFunIds that should be
   distinct from previous ones.  Previously we were only incrementing
   when defining new user-visible Ids.

   The main change is in HscTypes.extendInteractiveContext, which now
   alwyas bumps the ic_mod_index.  I also added a specialised
   extendInteractiveContextWithIds for the case where we are *only*
   adding new user-visible Ids.

Trac #9424
   In HscMain.hscDeclsWithLocations we were failing to use the
   *tidied* ClsInsts; but the un-tidied ones are LocalIds which
   causes a later ASSERT error.

   On the way I realised that, to behave consistently, the tcg_insts
   and tcg_fam_insts field of TcGblEnv should really only contain
   instances from the current GHCi command, not all the ones to date.
   That in turn meant I had to move the code for deleting replacement
   instances from addLocalInst, addLocalFamInst to
   HscTypes.extendInteractiveContext