[gn build] Port 2ca300f

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Differential Revision: https://phabricator.intern.facebook.com/D60250733

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/lib/CodeGen/GlobalISel/BUILD.gn

@@ -17,6 +17,7 @@ static_library("GlobalISel") {
     "CallLowering.cpp",
     "Combiner.cpp",
     "CombinerHelper.cpp",
+    "CombinerHelperCasts.cpp",
     "CombinerHelperVectorOps.cpp",
     "GIMatchTableExecutor.cpp",
     "GISelChangeObserver.cpp",