ISLE: move icmp rewrites to separate file.

[Kmeakin]

Move icmp-related rewrite rules from algebraic.isle to icmp.isle.
Also move icmp-related tests from algebraic.clif to icmp.clif.

[cfallin]

Taking a look!

[cfallin]

@Kmeakin it looks like this is still not quite a verbatim move, which is making it tricky for me to verify -- e.g., this rule

;; Optimize icmp-of-icmp.
(rule (simplify (ne ty
                      (uextend _ inner @ (icmp ty _ _ _))
                      (iconst _ (u64_from_imm64 0))))
      (subsume inner))

became

;; Optimize icmp-of-icmp:
(rule (simplify (ne ty (uextend _ inner @ (icmp ty _ _ _)) (iconst _ (u64_from_imm64 0))))
      (subsume inner))

I'm trying not to be too picky but I also find it much easier if code motion isn't mixed in with re-formatting, change of comments, etc.; if you don't mind, would you be able to redo the code-motion with a straight copy-paste, keeping things in the same order, then we can do other refactors later?

[cfallin]

Thanks!