shader_ir: Implement ST_S

This instruction writes to a memory buffer shared with threads within
the same work group. It is known as "shared" memory in GLSL.

src/video_core/shader/node.h

@@ -198,12 +198,13 @@ class PredicateNode;
 class AbufNode;
 class CbufNode;
 class LmemNode;
+class SmemNode;
 class GmemNode;
 class CommentNode;
 
 using NodeData =
     std::variant<OperationNode, ConditionalNode, GprNode, ImmediateNode, InternalFlagNode,
-                 PredicateNode, AbufNode, CbufNode, LmemNode, GmemNode, CommentNode>;
+                 PredicateNode, AbufNode, CbufNode, LmemNode, SmemNode, GmemNode, CommentNode>;
 using Node = std::shared_ptr<NodeData>;
 using Node4 = std::array<Node, 4>;
 using NodeBlock = std::vector<Node>;
@@ -536,6 +537,19 @@ private:
     Node address;
 };
 
+/// Shared memory node
+class SmemNode final {
+public:
+    explicit SmemNode(Node address) : address{std::move(address)} {}
+
+    const Node& GetAddress() const {
+        return address;
+    }
+
+private:
+    Node address;
+};
+
 /// Global memory node
 class GmemNode final {
 public: