Port flag-based ops to ISLE (AArch64) (#4942)

Ported the existing implementations of the following opcodes for AArch64
to ISLE:
- `Trueif`
- `Trueff`
- `Trapif`
- `Trapff`
- `Select`
- `Selectif`
- `SelectifSpectreGuard`

Copyright (c) 2022 Arm Limited

cranelift/interpreter/src/step.rs

@@ -942,15 +942,17 @@ where
         Opcode::Nearest => assign(Value::nearest(arg(0)?)?),
         Opcode::IsNull => unimplemented!("IsNull"),
         Opcode::IsInvalid => unimplemented!("IsInvalid"),
+        // `ctrl_ty` is `INVALID` for `Trueif` and `Trueff`, but both should
+        // return a 1-bit boolean value.
         Opcode::Trueif => choose(
             state.has_iflag(inst.cond_code().unwrap()),
-            Value::bool(true, ctrl_ty)?,
-            Value::bool(false, ctrl_ty)?,
+            Value::bool(true, types::B1)?,
+            Value::bool(false, types::B1)?,
         ),
         Opcode::Trueff => choose(
             state.has_fflag(inst.fp_cond_code().unwrap()),
-            Value::bool(true, ctrl_ty)?,
-            Value::bool(false, ctrl_ty)?,
+            Value::bool(true, types::B1)?,
+            Value::bool(false, types::B1)?,
         ),
         Opcode::Bitcast
         | Opcode::RawBitcast