x64: Migrate the return and fallthrough_return lowerings to ISLE (#4518)

https://github.com/bytecodealliance/wasmtime/pull/4518

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

@@ -26,7 +26,8 @@ use crate::{
         },
     },
     machinst::{
-        isle::*, InsnInput, InsnOutput, LowerCtx, MachAtomicRmwOp, VCodeConstant, VCodeConstantData,
+        isle::*, InsnInput, InsnOutput, LowerCtx, MachAtomicRmwOp, MachInst, VCodeConstant,
+        VCodeConstantData,
     },
 };
 use std::boxed::Box;
@@ -573,6 +574,11 @@ where
     fn atomic_rmw_op_to_mach_atomic_rmw_op(&mut self, op: &AtomicRmwOp) -> MachAtomicRmwOp {
         MachAtomicRmwOp::from(*op)
     }
+
+    #[inline]
+    fn gen_move(&mut self, ty: Type, dst: WritableReg, src: Reg) -> MInst {
+        MInst::gen_move(dst, src, ty)
+    }
 }
 
 // Since x64 doesn't have 8x16 shifts and we must use a 16x8 shift instead, we