Add source/opt/block_merge_util.* to BUILD.gn (#2344)

diff --git a/BUILD.gn b/BUILD.gn
index 69e3dc8..01044bf 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -452,6 +452,8 @@
     "source/opt/basic_block.h",
     "source/opt/block_merge_pass.cpp",
     "source/opt/block_merge_pass.h",
+    "source/opt/block_merge_util.cpp",
+    "source/opt/block_merge_util.h",
     "source/opt/build_module.cpp",
     "source/opt/build_module.h",
     "source/opt/ccp_pass.cpp",