Add doc and missing load check

src/ExtractTileOperations.cpp

@@ -239,6 +239,15 @@ class ExtractTileOperations : public IRMutator {
         return ProducerConsumer::make(amx_alloc, op->is_producer, body);
     }
 
+    Expr visit(const Load* op) override {
+        if (op->name == tile_name) {
+          // Any tile load will be matched elsewhere, so a load here means that
+          // the AMX tile is used outside of a tile instruction.
+          is_valid = false;
+        }
+        return IRMutator::visit(op);
+    }
+
     Stmt visit(const Store *op) override {
         if (op->name != tile_name) {
             const auto *load = op->value.as<Load>();

src/ExtractTileOperations.h

@@ -11,7 +11,8 @@
 namespace Halide {
 namespace Internal {
 
-/** TODO */
+/** Rewrite any AMX tile operations that have been stored in the AMXTile memory
+ * type as intrinsic calls, to be used in the X86 backend. */
 Stmt extract_tile_operations(const Stmt &s);
 
 }  // namespace Internal