Vector bitcast support (AArch64 & Interpreter) (#4820)

* Vector bitcast support (AArch64 & Interpreter)

Implemented support for `bitcast` on vector values for AArch64 and the
interpreter.

Also corrected the verifier to ensure that the size, in bits, of the input and
output types match for a `bitcast`, per the docs.

Copyright (c) 2022 Arm Limited

* `I128` same-type bitcast support

Copyright (c) 2022 Arm Limited

* Directly return input for 64-bit GPR<=>GPR bitcast

Copyright (c) 2022 Arm Limited

cranelift/interpreter/src/value.rs

@@ -326,7 +326,7 @@ impl Value for DataValue {
                     };
                     DataValue::int(val, t)?
                 }
-                (dv, t) if t.is_int() && dv.ty() == t => dv,
+                (dv, t) if (t.is_int() || t.is_float()) && dv.ty() == t => dv,
                 (dv, _) => unimplemented!("conversion: {} -> {:?}", dv.ty(), kind),
             },
             ValueConversionKind::Truncate(ty) => {