[AArch64] Force sink-fold=1 in ptrauth-load test while it's being iterated on. NFC.

The flag has been going back and forth for a few weeks, but this test
relies on some existing behavior that's not gated by the new flag.
Forcibly enable it;  we'll eventually be able to get rid of the flag.

rdar://117833011

Author: ahmedbougacha

llvm/test/CodeGen/AArch64/arm64e-ptrauth-load.ll

@@ -1,5 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
-; RUN: llc < %s -mtriple arm64e-apple-darwin -verify-machineinstrs | FileCheck %s
+; RUN: llc < %s -mtriple arm64e-apple-darwin -aarch64-enable-sink-fold=1 -verify-machineinstrs | FileCheck %s
+
+; FIXME: remove explicit sink-fold enable once it's stable.
+; rdar://117833011
 
 target datalayout = "e-m:o-i64:64-i128:128-n32:64-S128"