JIT: Don't always look up symbols for blocks

With tons of symbols, this results in noticeable stuttering, so
skip lookups if the perf dir option isn't set anyway.

Source/Core/Common/JitRegister.h

@@ -12,6 +12,7 @@ namespace JitRegister
 void Init(const std::string& perf_dir);
 void Shutdown();
 void RegisterV(const void* base_address, u32 code_size, const char* format, va_list args);
+bool IsEnabled();
 
 inline void Register(const void* base_address, u32 code_size, const char* format, ...)
 {