[gn build] Port 2011052

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/tools/llvm-reduce/BUILD.gn

@@ -25,6 +25,7 @@ executable("llvm-reduce") {
     "deltas/ReduceGlobalVarInitializers.cpp",
     "deltas/ReduceGlobalVars.cpp",
     "deltas/ReduceIRReferences.cpp",
+    "deltas/ReduceInstructionFlagsMIR.cpp",
     "deltas/ReduceInstructions.cpp",
     "deltas/ReduceInstructionsMIR.cpp",
     "deltas/ReduceMetadata.cpp",