[tree-ssa] copy prop vs. debugging

[Richard Henderson]

> Debuugging might seem wrong if a user variable
> isn't actually used in the last half of the program, but the value
> changes all the time because we are reusing the stack slot... Or is that
> OK?  It would complicate things a little bit to make sure that doesnt
> happen.

It's ok outside of -O0.  We get that effect anyway from variables
in registers that get re-used after they're dead.

The Most Proper thing to do in this case is to make sure that the
dwarf2 location lists express the fact that the user variable is
live between points A and B, and not across the entire scope.


r~