Implement iabs in ISLE (AArch64) (#4399)

* Implement `iabs` in ISLE (AArch64)

Converts the existing implementation of `iabs` for AArch64 into ISLE,
and fixes support for `iabs` on scalar values.

Copyright (c) 2022 Arm Limited.

* Improve scalar `iabs` implementation.

Also introduces `CSNeg` instruction.

Copyright (c) 2022 Arm Limited

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

@@ -1941,17 +1941,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             panic!("ALU+imm and ALU+carry ops should not appear here!");
         }
 
-        Opcode::Iabs => {
-            let rd = get_output_reg(ctx, outputs[0]).only_reg().unwrap();
-            let rn = put_input_in_reg(ctx, inputs[0], NarrowValueMode::None);
-            let ty = ty.unwrap();
-            ctx.emit(Inst::VecMisc {
-                op: VecMisc2::Abs,
-                rd,
-                rn,
-                size: VectorSize::from_ty(ty),
-            });
-        }
+        Opcode::Iabs => implemented_in_isle(ctx),
         Opcode::AvgRound => {
             let ty = ty.unwrap();