[MemCpyOpt] Avoid double invalidation (NFCI)

The removal of the cpy instruction is left to the caller of
performCallSlotOptzn(), including the invalidation of MD. Both
call-sites already do this.

Also handle incrementation of NumMemCpyInstr consistently at the
call-site. One of the call-site was already doing this, which
ended up incrementing the statistic twice.

This fix was part of D26739.