[trans-mem] avoid transforming already optimized memory variants

[Aldy Hernandez]

> If you're going to do this, are you going to incorporate this knowledge
> into the optimization pass?  Or is the fact that we already have an
> address instead of a memory too annoying?

Do you mean not accumulating any optimized stores/loads variants if
we're not going to (re)optimize them (tm_memopt_accumulate_memops)?

I suppose we should get tm_memopt_accumulate_memops() to use
is_tm_simple* to avoid doing unecessary work.

Perhaps we should avoid global memory optimizations altogether for functions
that have hand-coded RaR, RfW, etc?