Remove reg_universe method from MachBackend and MachInst

cranelift/codegen/src/isa/x64/mod.rs

@@ -50,7 +50,7 @@ impl X64Backend {
         // block layout and finalizes branches. The result is ready for binary emission.
         let emit_info = EmitInfo::new(flags.clone(), self.x64_flags.clone());
         let abi = Box::new(abi::X64ABICallee::new(&func, flags)?);
-        compile::compile::<Self>(&func, self, abi, self.reg_universe(), emit_info)
+        compile::compile::<Self>(&func, self, abi, &self.reg_universe, emit_info)
     }
 }
 
@@ -102,10 +102,6 @@ impl MachBackend for X64Backend {
         &self.triple
     }
 
-    fn reg_universe(&self) -> &RealRegUniverse {
-        &self.reg_universe
-    }
-
     fn unsigned_add_overflow_condition(&self) -> IntCC {
         // Unsigned `<`; this corresponds to the carry flag set on x86, which
         // indicates an add has overflowed.