x64: port atomic_cas to ISLE (#4223)

cranelift/codegen/src/isa/x64/lower/isle.rs

@@ -547,6 +547,12 @@ where
     fn zero_offset(&mut self) -> Offset32 {
         Offset32::new(0)
     }
+
+    #[inline]
+    fn rax(&mut self) -> WritableGpr {
+        let gpr = Gpr::new(regs::rax()).unwrap();
+        WritableGpr::from_reg(gpr)
+    }
 }
 
 // Since x64 doesn't have 8x16 shifts and we must use a 16x8 shift instead, we