tile: allow memcpy(p, p, n) without corrupting memory at "p"

tile: allow memcpy(p, p, n) without corrupting memory at "p"

Although this is not required by the definition of memcpy(),
in practice this sort of thing does happen, and it's easy to make
the code robust by doing nothing in this case.  (Since structure
copy causes the compiler to emit a memcpy, in the case where the
target structure is the same as the destination, we were seeing
corruption.)