Add SVE-specific cost test for VLS

Author: gbossu

llvm/test/Analysis/CostModel/AArch64/shuffle-extract.ll

@@ -1,6 +1,9 @@
 ; NOTE: Assertions have been autogenerated by utils/update_analyze_test_checks.py
 ; RUN: opt < %s -mtriple=aarch64--linux-gnu -passes="print<cost-model>" -cost-kind=all 2>&1 -disable-output | FileCheck %s
 
+; This tests the cost of fixed-length subvector extracts for NEON.
+; For the SVE equivalent test, see sve-vls-shuffle-extract.ll
+
 target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
 
 define void @extract_half() {