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

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

cranelift/codegen/src/isa/x64/inst.isle

@@ -1486,8 +1486,34 @@
                   r
                   (OperandSize.Size32)))
 
+
 ;;;; Helpers for Emitting Loads ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
+;; Generate a move between two registers.
+(decl gen_move (Type WritableReg Reg) MInst)
+(extern constructor gen_move gen_move)
+
+;; Copy a return value to a set of registers.
+(decl copy_to_regs (WritableValueRegs Value) Unit)
+(rule (copy_to_regs dsts val @ (value_type ty))
+      (let ((srcs ValueRegs (put_in_regs val)))
+        (copy_to_regs_range ty (value_regs_range srcs) dsts srcs)))
+
+;; Helper for `copy_to_regs` that uses a range to index into the reg/value
+;; vectors. Fails for the empty range.
+(decl copy_to_regs_range (Type Range WritableValueRegs ValueRegs) Unit)
+
+(rule (copy_to_regs_range ty (range_singleton idx) dsts srcs)
+      (let ((dst WritableReg (writable_regs_get dsts idx))
+            (src Reg (value_regs_get srcs idx)))
+        (emit (gen_move ty dst src))))
+
+(rule (copy_to_regs_range ty (range_unwrap head tail) dsts srcs)
+      (let ((dst WritableReg (writable_regs_get dsts head))
+            (src Reg (value_regs_get srcs head))
+            (_ Unit (emit (gen_move ty dst src))))
+        (copy_to_regs_range ty tail dsts srcs)))
+
 ;; Helper for constructing a LoadExtName instruction.
 (decl load_ext_name (ExternalName i64) Reg)
 (rule (load_ext_name extname offset)