[tree-ssa] Lazy updating of stmt operands

[Chris Lattner]

> > hardly; you still must scan all statements to find the uses, so I don't
> > see where you would want to get the extra efficiency.
> >
> Scanning stmts is very cheap.

You must not have run across programs that have PHI nodes with thousands
of operands...

> The uses/defs are all cached.

Don't caches take space?

> And if you do need it over a chain of optimizations, do it once, and
> then keep the info up to date/.  You'll be a lot better off I think

Isn't that the whole idea?

-Chris

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