[gn build] Port dcb8348

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/lib/Target/AMDGPU/BUILD.gn

@@ -237,6 +237,7 @@ static_library("LLVMAMDGPUCodeGen") {
     "SIMachineScheduler.cpp",
     "SIMemoryLegalizer.cpp",
     "SIModeRegister.cpp",
+    "SIModeRegisterDefaults.cpp",
     "SIOptimizeExecMasking.cpp",
     "SIOptimizeExecMaskingPreRA.cpp",
     "SIOptimizeVGPRLiveRange.cpp",