[llvm] [AArch64][SME] Conditionally do smstart/smstop (PR #77113)

[Sander de Smalen via llvm-commits]

================
@@ -208,6 +208,8 @@ class AArch64FunctionInfo final : public MachineFunctionInfo {
 
   int64_t StackProbeSize = 0;
 
+  Register PStateSMReg = MCRegister::NoRegister;
----------------
sdesmalen-arm wrote:

nit: Can you add a comment describing what this Register holds and when this value is defined?

https://github.com/llvm/llvm-project/pull/77113