[tree-ssa] RFC: Making control flow more explicit

[law@redhat.com]

In message <1060704909.6238.10.camel@p4>, Andrew MacLeod writes:
 >We let the cfg cleanup routines actually remove control flow most of the
 >time now, do we not? And I beleive you also wanted a lighter-weight DCE.
 >So perhaps we ought to invoke DCE with a flag indicating whether to
 >consider control flow or not.
I actually played with that as well :-)   It's not terribly difficult
to implement and did mitigate the cost of doing full control dependence
analysis to the last pass.

I nearly went with that solution -- but I wanted to benchmark that
approach against an even simpler DCE implementation before recommending
one approach over the other.

 >I suspect we'd only need the flow considered during the very last call
 >before we go back to normal form.
Yup.

Jeff