x64: port some atomics to ISLE (#4212)

* x64: port `fence` to ISLE
* x64: port `atomic_load` to ISLE
* x64: port `atomic_store` to ISLE

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

@@ -524,15 +524,6 @@ impl Inst {
         }
     }
 
-    /// A convenience function to be able to use a RegMem as the source of a move.
-    pub(crate) fn mov64_rm_r(src: RegMem, dst: Writable<Reg>) -> Inst {
-        src.assert_regclass_is(RegClass::Int);
-        match src {
-            RegMem::Reg { reg } => Self::mov_r_r(OperandSize::Size64, reg, dst),
-            RegMem::Mem { addr } => Self::mov64_m_r(addr, dst),
-        }
-    }
-
     pub(crate) fn mov_r_m(size: OperandSize, src: Reg, dst: impl Into<SyntheticAmode>) -> Inst {
         debug_assert!(src.class() == RegClass::Int);
         Inst::MovRM {