Port Fence, IsNull/IsInvalid & Debugtrap to ISLE (AArch64) (#4548)

Ported the existing implementation of the following Opcodes for AArch64
to ISLE:
- `Fence`
- `IsNull`
- `IsInvalid`
- `Debugtrap`

Copyright (c) 2022 Arm Limited

cranelift/codegen/src/isa/aarch64/lower.isle

@@ -1699,3 +1699,29 @@
       (let ((low_half Reg (uqxtn x (lane_size ty)))
             (result Reg (uqxtn2 low_half y (lane_size ty))))
         result))
+
+;;;; Rules for `Fence` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule (lower (fence))
+      (let ((_ Unit (emit (MInst.Fence))))
+            (output_none)))
+
+;;;; Rules for `IsNull` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule (lower (has_type out_ty (is_null x @ (value_type ty))))
+      (with_flags (cmp_imm (operand_size ty) x (u8_into_imm12 0))
+                  (materialize_bool_result
+                   (ty_bits out_ty) (Cond.Eq))))
+
+;;;; Rules for `IsInvalid` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule (lower (has_type out_ty (is_invalid x @ (value_type ty))))
+      (with_flags (cmn_imm (operand_size ty) x (u8_into_imm12 1))
+                  (materialize_bool_result
+                   (ty_bits out_ty) (Cond.Eq))))
+
+;;;; Rules for `Debugtrap` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule (lower (debugtrap))
+      (let ((_ Unit (emit (MInst.Brk))))
+            (output_none)))