div to shift for positive power of 2 divs

[KGrewal1]

An attempt at adding isle opts for turning division by powers of two into ushr or sshr bit shifts

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[fitzgen]

I don't think this is viable yet because the e-graph cannot rewrite side effecting instructions in general, see #5908 for more information.

I suspect that if you attempted to write a filetest exercising this rule, you'd find that it never fires because we never put division instructions into the egraph, and leave them in the "skeleton" outside of the graph.

[KGrewal1]

Ah ok: looking through that issue seems like the current div simplifications in that file also currently do nothing for the same reason...

[elliottt]

@KGrewal1 I'm going to close this PR for now, as it's not possible to implement these rules with the constraints of our current e-graph implementation. We can re-open this PR if that changes in the future.