[StackProtector] Add test for atomicrmw xchg (NFC)

This is an opt based test because usually AtomicExpand will
convert it to an integer atomicrmw first.

Author: nikic

llvm/test/CodeGen/X86/stack-protector-atomicrmw-xchg.ll

@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -mtriple=x86_64-unknown-linux-gnu -passes=stack-protector < %s | FileCheck %s
+
+define void @atomicrmw_xchg(ptr %p) sspstrong {
+; CHECK-LABEL: define void @atomicrmw_xchg(
+; CHECK-SAME: ptr [[P:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:    [[A:%.*]] = alloca i64, align 8
+; CHECK-NEXT:    [[TMP1:%.*]] = atomicrmw xchg ptr [[P]], ptr [[A]] acquire, align 8
+; CHECK-NEXT:    ret void
+;
+  %a = alloca i64
+  atomicrmw xchg ptr %p, ptr %a acquire
+  ret void
+}