[PR30643] restore memory cse

[Paolo Bonzini]

> Later on, with the fwprop pass, we completely lost the ability to
> replace memory accesses with known values, since AFAICT DF, used by
> fwprop, only propagates information on REGs and SUBREGs.
> 
> So the patch for the trunk restores the code that we used to have to
> CSE memory.

Sigh.  I also don't see any other thing to do.

But these are all missed tree optimization opportunities.  Can you
please turn the bug into an enhancement request when the patch is
approved, instead of closing it?

Thanks,

Paolo