[tree-ssa] Lazy updating of stmt operands

[Chris Lattner]

Jeff Law wrote:

> While it's nice to think about a world where you always have immediate
> uses, you end up with FUD chains if you do that -- with ultimately a
> tangled nest of pointers that is bloody impossible to work with in the
> real world.

Uh, no.  Here's a counter-example:
http://llvm.cs.uiuc.edu/

Having efficient, transparently & efficiently updated immediate use
information is extremely valuable for a wide variety of optimizations.

-Chris

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