cranelift: Port most of simple_preopt.rs over to the peepmatic DSL

This ports all of the identity, no-op, simplification, and canonicalization
related optimizations over from being hand-coded to the `peepmatic` DSL. This
does not handle the branch-to-branch optimizations or most of the
divide-by-constant optimizations.

cranelift/peepmatic/src/lib.rs

@@ -160,6 +160,6 @@ mod tests {
 
     #[test]
     fn compile_preopt() {
-        assert_compiles("examples/preopt.peepmatic");
+        assert_compiles("../codegen/src/preopt.peepmatic");
     }
 }