Fix crash lowering stack guard on OpenBSD/aarch64. (#125416)

TargetLoweringBase::getIRStackGuard refers to a platform-specific guard
variable. Before this change, TargetLoweringBase::getSDagStackGuard only
referred to a different variable.

This means that SelectionDAGBuilder's getLoadStackGuard does not get
memory operands. However, AArch64InstrInfo::expandPostRAPseudo assumes
that the passed MachineInstr has nonzero memoperands, causing a
segfault.

We have two possible options here: either disabling the LOAD_STACK_GUARD
node entirely in AArch64TargetLowering::useLoadStackGuardNode or just
making the platform-specific values match across TargetLoweringBase.
Here, we try the latter.

llvm/lib/CodeGen/TargetLoweringBase.cpp

diff --git a/llvm/lib/CodeGen/TargetLoweringBase.cpp b/llvm/lib/CodeGen/TargetLoweringBase.cpp
index 3da66a4..91ae904 100644
--- a/llvm/lib/CodeGen/TargetLoweringBase.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringBase.cpp
@@ -2005,6 +2005,9 @@
 // Currently only support "standard" __stack_chk_guard.
 // TODO: add LOAD_STACK_GUARD support.
 Value *TargetLoweringBase::getSDagStackGuard(const Module &M) const {
+  if (getTargetMachine().getTargetTriple().isOSOpenBSD()) {
+    return M.getNamedValue("__guard_local");
+  }
   return M.getNamedValue("__stack_chk_guard");
 }