video_core/shader/ast: Make ASTManager::Print a const member function

Given all visiting functions never modify the nodes, we can trivially
make this a const member function.

src/video_core/shader/ast.cpp

@@ -333,7 +333,7 @@ public:
         inner += fmt::format("{}({}) -> break;\n", Indent(), expr_parser.GetResult());
     }
 
-    void Visit(ASTNode& node) {
+    void Visit(const ASTNode& node) {
         std::visit(*this, *node->GetInnerData());
     }
 
@@ -364,7 +364,7 @@ private:
     static constexpr std::string_view spaces{"                                    "};
 };
 
-std::string ASTManager::Print() {
+std::string ASTManager::Print() const {
     ASTPrinter printer{};
     printer.Visit(main_node);
     return printer.GetResult();

src/video_core/shader/ast.h

@@ -328,7 +328,7 @@ public:
 
     void InsertReturn(Expr condition, bool kills);
 
-    std::string Print();
+    std::string Print() const;
 
     void Decompile();