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_inst.rs

@@ -254,9 +254,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
 
         Opcode::AtomicStore => implemented_in_isle(ctx),
 
-        Opcode::Fence => {
-            ctx.emit(Inst::Fence {});
-        }
+        Opcode::Fence => implemented_in_isle(ctx),
 
         Opcode::StackLoad
         | Opcode::StackStore
@@ -399,34 +397,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             materialize_bool_result(ctx, insn, rd, cond);
         }
 
-        Opcode::IsNull | Opcode::IsInvalid => {
-            // Null references are represented by the constant value 0; invalid references are
-            // represented by the constant value -1. See `define_reftypes()` in
-            // `meta/src/isa/x86/encodings.rs` to confirm.
-            let rd = get_output_reg(ctx, outputs[0]).only_reg().unwrap();
-            let rn = put_input_in_reg(ctx, inputs[0], NarrowValueMode::None);
-            let ty = ctx.input_ty(insn, 0);
-            let (alu_op, const_value) = match op {
-                Opcode::IsNull => {
-                    // cmp rn, #0
-                    (ALUOp::SubS, 0)
-                }
-                Opcode::IsInvalid => {
-                    // cmn rn, #1
-                    (ALUOp::AddS, 1)
-                }
-                _ => unreachable!(),
-            };
-            let const_value = ResultRSEImm12::Imm12(Imm12::maybe_from_u64(const_value).unwrap());
-            ctx.emit(alu_inst_imm12(
-                alu_op,
-                ty,
-                writable_zero_reg(),
-                rn,
-                const_value,
-            ));
-            materialize_bool_result(ctx, insn, rd, Cond::Eq);
-        }
+        Opcode::IsNull | Opcode::IsInvalid => implemented_in_isle(ctx),
 
         Opcode::Copy => {
             let rd = get_output_reg(ctx, outputs[0]).only_reg().unwrap();
@@ -546,9 +517,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             }
         }
 
-        Opcode::Debugtrap => {
-            ctx.emit(Inst::Brk);
-        }
+        Opcode::Debugtrap => implemented_in_isle(ctx),
 
         Opcode::Trap | Opcode::ResumableTrap => implemented_in_isle(ctx),