Cranelift: don't emit inside lowering rules for aarch64 (#4572)

* Cranelift: Don't `emit` inside lowering rules in aarch64

The lowering rules should be "pure" and side-effect free, using helpers defined
in `inst.isle` to perform actual side effects like emitting instructions.

* Cranelift: use 80 width for section separators in aarch64 lowering rules

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

@@ -1905,6 +1905,16 @@
 (decl uqxtn2 (Reg Reg ScalarSize) Reg)
 (rule (uqxtn2 x y size) (vec_rr_narrow_high (VecRRNarrowOp.Uqxtn) x y size))
 
+;; Helper for generating `fence` instructions.
+(decl aarch64_fence () SideEffectNoResult)
+(rule (aarch64_fence)
+      (SideEffectNoResult.Inst (MInst.Fence)))
+
+;; Helper for generating `brk` instructions.
+(decl brk () SideEffectNoResult)
+(rule (brk)
+      (SideEffectNoResult.Inst (MInst.Brk)))
+
 ;; Helper for generating `addp` instructions.
 (decl addp (Reg Reg VectorSize) Reg)
 (rule (addp x y size) (vec_rrr (VecALUOp.Addp) x y size))

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

@@ -1700,28 +1700,26 @@
             (result Reg (uqxtn2 low_half y (lane_size ty))))
         result))
 
-;;;; Rules for `Fence` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;; Rules for `Fence` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (rule (lower (fence))
-      (let ((_ Unit (emit (MInst.Fence))))
-            (output_none)))
+      (side_effect (aarch64_fence)))
 
-;;;; Rules for `IsNull` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;; 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` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;; 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` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;;;; Rules for `Debugtrap` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (rule (lower (debugtrap))
-      (let ((_ Unit (emit (MInst.Brk))))
-            (output_none)))
+      (side_effect (brk)))