[fixup] Add a comment how we depend on the implementation of hasReservedCallFrame

Author: momchil-velikov

llvm/lib/Target/AArch64/AArch64FrameLowering.cpp

@@ -479,6 +479,11 @@ bool AArch64FrameLowering::hasFP(const MachineFunction &MF) const {
 /// included as part of the stack frame.
 bool
 AArch64FrameLowering::hasReservedCallFrame(const MachineFunction &MF) const {
+  // The stack probing code for the dynamically allocated outgoing arguments
+  // area assumes that the stack is probed at the top - either by the prologue
+  // code, which issues a probe if `hasVarSizedObjects` return true, or by the
+  // most recent variable-sized object allocation. Changing the condition here
+  // may need to be followed up by changes to the probe issuing logic.
   return !MF.getFrameInfo().hasVarSizedObjects();
 }