[tree-ssa] copy prop vs. debugging

[Jason Merrill <jason at redhat dot com>]

When a temporary is copied into a variable, any dominated uses of that
variable are replaced with uses of the temporary.  If all uses are
dominated by such a copy, the variable is removed.  This obviously hurts
debugging.

This isn't a major problem in the current tree, but a patch I'm working
on to always evaluate non-trivial expressions into temps causes it to
happen much more frequently; now pretty much any modification of a user
variable is a copy.

I can think of two ways to address this:

1) When coalescing, try to replace temps with user variables which are
   copied from them.  This is an incomplete solution, but might be simpler
   than:
2) Remember the dead copies, make a list of which temporary holds the value
   of a variable at any given time, and feed this list to the debugging
   backend.

Thoughts?  Is there any live range splitting code in gcc these days?

Jason