Port vconst to ISLE (AArch64) (#4750)

* Port `vconst` to ISLE (AArch64)

Ported the existing implementation of `vconst` to ISLE for AArch64, and
added support for 64-bit vector constants.

Also introduced 64-bit `vconst` support to the interpreter.

Copyright (c) 2022 Arm Limited

* Replace if-chains with match statements

Copyright (c) 2022 Arm Limited

cranelift/codegen/src/isa/aarch64/lower.rs

@@ -8,7 +8,6 @@
 //! - Floating-point immediates (FIMM instruction).
 
 use super::lower_inst;
-use crate::data_value::DataValue;
 use crate::ir::condcodes::{FloatCC, IntCC};
 use crate::ir::types::*;
 use crate::ir::Inst as IRInst;
@@ -94,13 +93,6 @@ pub(crate) fn input_to_shiftimm(
     input_to_const(ctx, input).and_then(ShiftOpShiftImm::maybe_from_shift)
 }
 
-pub(crate) fn const_param_to_u128(ctx: &mut Lower<Inst>, inst: IRInst) -> Option<u128> {
-    match ctx.get_immediate(inst) {
-        Some(DataValue::V128(bytes)) => Some(u128::from_le_bytes(bytes)),
-        _ => None,
-    }
-}
-
 /// How to handle narrow values loaded into registers; see note on `narrow_mode`
 /// parameter to `put_input_in_*` below.
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]