m32c vs loop-opts - possible work-around

[DJ Delorie]

If loop optimization assumes sizeof(void *) == sizeof(size_t) and
won't work correctly otherwise, perhaps a check for that condition in
gate_tree_ssa_loop_ivopts() would be appropriate?  I thought of just
disabling that flag in m32c.c, but I figured a more generic solution
would make more sense, and occasionally get the attention of the loop
maintainers to remind them of the limitation.