[llvm] [NFC][TableGen] Add IfGuardEmitter and adopt it in InstrInfoEmitter (PR #168616)

[Rahul Joshi via llvm-commits]

================
@@ -46,6 +46,23 @@ class IfDefEmitter {
   bool LateUndef;
 };
 
+// Simple RAII helper for emitting #if guard. It emits:
+// #if <Condition>
+// #endif // <Condition>
+class IfGuardEmitter {
+public:
+  IfGuardEmitter(raw_ostream &OS, StringRef Condition)
+      : Condition(Condition.str()), OS(OS) {
+    OS << "#if " << Condition << "\n\n";
+  }
+
+  ~IfGuardEmitter() { OS << "\n#endif // " << Condition << "\n\n"; }
+
+private:
+  std::string Condition;
----------------
jurahul wrote:

I anticipate use cases where condition can be a temporary value just like in other emitters, so creating a copy. Also, for uniformity with other emitters, creating a copy.

https://github.com/llvm/llvm-project/pull/168616