Fixed build warning.

llvm-svn: 100244
diff --git a/llvm/utils/TableGen/ARMDecoderEmitter.cpp b/llvm/utils/TableGen/ARMDecoderEmitter.cpp
index 50136bc..12c753b 100644
--- a/llvm/utils/TableGen/ARMDecoderEmitter.cpp
+++ b/llvm/utils/TableGen/ARMDecoderEmitter.cpp
@@ -300,7 +300,7 @@
   FilterChooser &getVariableFC() {
     assert(NumFiltered == 1);
     assert(FilterChooserMap.size() == 1);
-    return *(FilterChooserMap.find(-1)->second);
+    return *(FilterChooserMap.find((unsigned)-1)->second);
   }
 
   Filter(const Filter &f);