video_core/shader/ast: Make ShowCurrentState() and SanityCheck() const member functions

These can also trivially be made const member functions, with the
addition of a few consts.

src/video_core/shader/ast.cpp

@@ -560,13 +560,13 @@ bool ASTManager::DirectlyRelated(const ASTNode& first, const ASTNode& second) co
     return min->GetParent() == max->GetParent();
 }
 
-void ASTManager::ShowCurrentState(std::string_view state) {
+void ASTManager::ShowCurrentState(std::string_view state) const {
     LOG_CRITICAL(HW_GPU, "\nState {}:\n\n{}\n", state, Print());
     SanityCheck();
 }
 
-void ASTManager::SanityCheck() {
-    for (auto& label : labels) {
+void ASTManager::SanityCheck() const {
+    for (const auto& label : labels) {
         if (!label->GetParent()) {
             LOG_CRITICAL(HW_GPU, "Sanity Check Failed");
         }

src/video_core/shader/ast.h

@@ -332,9 +332,9 @@ public:
 
     void Decompile();
 
-    void ShowCurrentState(std::string_view state);
+    void ShowCurrentState(std::string_view state) const;
 
-    void SanityCheck();
+    void SanityCheck() const;
 
     void Clear();