Port Fcopysign..FcvtToSintSat to ISLE (AArch64) (#4753)

* Port `Fcopysign`..``FcvtToSintSat` to ISLE (AArch64)

Ported the existing implementations of the following opcodes to ISLE on
AArch64:
- `Fcopysign`
  - Also introduced missing support for `fcopysign` on vector values, as
    per the docs.
  - This introduces the vector encoding for the `SLI` machine
    instruction.
- `FcvtToUint`
- `FcvtToSint`
- `FcvtFromUint`
- `FcvtFromSint`
- `FcvtToUintSat`
- `FcvtToSintSat`

Copyright (c) 2022 Arm Limited

* Document helpers and abstract conversion checks

cranelift/interpreter/src/step.rs

@@ -808,7 +808,19 @@ where
         }
         Opcode::Fneg => assign(Value::neg(arg(0)?)?),
         Opcode::Fabs => assign(Value::abs(arg(0)?)?),
-        Opcode::Fcopysign => binary(Value::copysign, arg(0)?, arg(1)?)?,
+        Opcode::Fcopysign => {
+            let arg0 = extractlanes(&arg(0)?, ctrl_ty)?;
+            let arg1 = extractlanes(&arg(1)?, ctrl_ty)?;
+
+            assign(vectorizelanes(
+                &arg0
+                    .into_iter()
+                    .zip(arg1.into_iter())
+                    .map(|(x, y)| V::copysign(x, y))
+                    .collect::<ValueResult<SimdVec<V>>>()?,
+                ctrl_ty,
+            )?)
+        }
         Opcode::Fmin => assign(match (arg(0)?, arg(1)?) {
             (a, _) if a.is_nan()? => a,
             (_, b) if b.is_nan()? => b,