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/filetests/filetests/runtests/ref64-invalid-null.clif

@@ -0,0 +1,42 @@
+; Tests for platforms with 64-bit references.
+test run
+target aarch64
+target x86_64
+target s390x
+
+function %is_null_true_r64() -> b1 {
+block0:
+  v0 = null.r64
+  v1 = is_null v0
+  return v1
+}
+; run: %is_null_true_r64() == true
+
+function %is_null_r64(i64) -> b1 {
+block0(v0: i64):
+  v1 = raw_bitcast.r64 v0
+  v2 = is_null v1
+  return v2
+}
+; run: %is_null_r64(256347) == false
+; run: %is_null_r64(-1) == false
+; run: %is_null_r64(0) == true
+
+function %is_invalid_r64(i64) -> b1 {
+block0(v0: i64):
+  v1 = raw_bitcast.r64 v0
+  v2 = is_invalid v1
+  return v2
+}
+; run: %is_invalid_r64(0xffffffffffffffff) == true
+; run: %is_invalid_r64(-1) == true
+; run: %is_invalid_r64(256347) == false
+; run: %is_invalid_r64(0) == false
+
+function %is_invalid_null_r64() -> b1 {
+block0:
+  v0 = null.r64
+  v1 = is_invalid v0
+  return v1
+}
+; run: %is_invalid_null_r64() == false