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

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

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

@@ -923,29 +923,12 @@ fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
         | Opcode::AtomicStore
         | Opcode::Fence
         | Opcode::FuncAddr
-        | Opcode::SymbolValue => {
+        | Opcode::SymbolValue
+        | Opcode::FallthroughReturn
+        | Opcode::Return => {
             implemented_in_isle(ctx);
         }
 
-        Opcode::FallthroughReturn | Opcode::Return => {
-            for i in 0..ctx.num_inputs(insn) {
-                let src_reg = put_input_in_regs(ctx, inputs[i]);
-                let retval_reg = ctx.retval(i);
-                let ty = ctx.input_ty(insn, i);
-                assert!(src_reg.len() == retval_reg.len());
-                let (_, tys) = Inst::rc_for_type(ty)?;
-                for ((&src, &dst), &ty) in src_reg
-                    .regs()
-                    .iter()
-                    .zip(retval_reg.regs().iter())
-                    .zip(tys.iter())
-                {
-                    ctx.emit(Inst::gen_move(dst, src, ty));
-                }
-            }
-            // N.B.: the Ret itself is generated by the ABI.
-        }
-
         Opcode::Call | Opcode::CallIndirect => {
             let caller_conv = ctx.abi().call_conv();
             let (mut abi, inputs) = match op {