Convert swizzle to ISLE (AArch64) (#4400)

Converted the implementation of `swizzle` for AArch64 to ISLE.

Copyright (c) 2022 Arm Limited

cranelift/codegen/src/isa/aarch64/inst.isle

@@ -1537,6 +1537,13 @@
             (_ Unit (emit (MInst.VecMisc op dst src size))))
         dst))
 
+;; Helper for emitting `MInst.VecTbl` instructions.
+(decl vec_tbl (Reg Reg bool) Reg)
+(rule (vec_tbl rn rm is_extension)
+      (let ((dst WritableReg (temp_writable_reg $I8X16))
+            (_ Unit (emit (MInst.VecTbl dst rn rm is_extension))))
+        dst))
+
 ;; Helper for emitting `MInst.VecRRRLong` instructions.
 (decl vec_rrr_long (VecRRRLongOp Reg Reg bool) Reg)
 (rule (vec_rrr_long op src1 src2 high_half)

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

@@ -99,6 +99,11 @@
           (add_with_flags_paired $I64 x_lo y_lo)
           (adc_paired $I64 x_hi y_hi))))
 
+;;;; Rules for `swizzle` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+
+(rule (lower (has_type vec_i128_ty (swizzle rn rm)))
+      (vec_tbl rn rm #f))
+
 ;;;; Rules for `iadd_pairwise` ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (rule (lower (has_type $I16X8 (iadd_pairwise (swiden_low x) (swiden_high y))))

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

@@ -1190,18 +1190,7 @@ pub(crate) fn lower_insn_to_regs<C: LowerCtx<I = Inst>>(
             });
         }
 
-        Opcode::Swizzle => {
-            let rd = get_output_reg(ctx, outputs[0]).only_reg().unwrap();
-            let rm = put_input_in_reg(ctx, inputs[1], NarrowValueMode::None);
-            let rn = put_input_in_reg(ctx, inputs[0], NarrowValueMode::None);
-
-            ctx.emit(Inst::VecTbl {
-                rd,
-                rn,
-                rm,
-                is_extension: false,
-            });
-        }
+        Opcode::Swizzle => implemented_in_isle(ctx),
 
         Opcode::Isplit => {
             let input_ty = ctx.input_ty(insn, 0);