tgsi: add assertion to verify legal register file

This assertion fails with piglit glsl-vs-mov-after-deref test
because we're double freeing the memory.  It seems there's some
confusion between what's placed in the hash table and what isn't.

src/gallium/auxiliary/tgsi/tgsi_sanity.c

@@ -515,6 +515,7 @@ regs_hash_destroy(struct cso_hash *hash)
    while (!cso_hash_iter_is_null(iter)) {
       scan_register *reg = (scan_register *)cso_hash_iter_data(iter);
       iter = cso_hash_erase(hash, iter);
+      assert(reg->file < TGSI_FILE_COUNT);
       FREE(reg);
    }
    cso_hash_delete(hash);