Add a few more integer ops "under consideration".

[sunfishcode]

No description provided.

[jfbastien]

I'm not sure I agree with all of them, but there doesn't seem to be much harm in considering them. I'm especially wary of adding things which the macro layer would catch, or which the compiler can trivially peephole, but we can have that discussion elsewhere.

[sunfishcode]

I agree that it's not always clear which operators should be included. One possible criteria is that instructions whose expansion would otherwise reference any of the operands multiple times have some value. Clrs, Abs, and SExt all satisfy this criteria. On the other hand, Neg and Bang don't, but those operators are being considered for the opposite reason, because they provide orthogonal fundamentals from which other operations can be synthesized.

[sunfishcode]

This is subsumed by #121.