[tree-ssa] reaching def. question

[law@redhat.com]

In message <1079543327.9691.15.camel@p4>, Andrew MacLeod writes:
 >I also think at some point we need to stop taking all versions of an SSA
 >variable out of, and back into SSA, and focus just on the ones we need.
 >Things like copyrenaming which changes the base variable of an
 >ssa_version could wreak havoc on such activity. 
I certainly can't disagree with this.  We do this in response to jump
threading changing the dominator tree -- I wouldn't be terribly surprised
if we could come up with an incremental way to handle this which didn't
require taking all the versions of affected variables out of SSA form.

Jeff