Add Hydra mid-end ruleset

Author: bongjunj

cranelift/codegen/meta/src/isle.rs

@@ -116,6 +116,7 @@ pub fn get_isle_compilations(
                     src_opts.join("spaceship.isle"),
                     src_opts.join("spectre.isle"),
                     src_opts.join("vector.isle"),
+                    src_opts.join("hydra.isle"),
                 ],
                 untracked_inputs: vec![numerics_isle.clone(), clif_opt_isle],
             },