Kernel/Threads: Don't clear the CPU instruction cache when performing a context switch from an idle thread into a thread in the same process.

We were unnecessarily clearing the cache when going from Process A -> Idle -> Process A, this caused extreme performance regressions.

src/core/hle/kernel/thread.cpp

@@ -171,6 +171,8 @@ static void SwitchContext(Thread* new_thread) {
         // Cancel any outstanding wakeup events for this thread
         CoreTiming::UnscheduleEvent(ThreadWakeupEventType, new_thread->callback_handle);
 
+        auto previous_process = Kernel::g_current_process;
+
         current_thread = new_thread;
 
         ready_queue.remove(new_thread->current_priority, new_thread);
@@ -179,7 +181,7 @@ static void SwitchContext(Thread* new_thread) {
         Core::CPU().LoadContext(new_thread->context);
         Core::CPU().SetCP15Register(CP15_THREAD_URO, new_thread->GetTLSAddress());
 
-        if (!previous_thread || previous_thread->owner_process != current_thread->owner_process) {
+        if (previous_process != current_thread->owner_process) {
             Kernel::g_current_process = current_thread->owner_process;
             Memory::current_page_table = &Kernel::g_current_process->vm_manager.page_table;
             // We have switched processes and thus, page tables, clear the instruction cache so we