Silence MSVC error C2398

Reported by http://lab.llvm.org:8011/builders/llvm-clang-x86_64-expensive-checks-win/builds/6000/steps/build-unified-tree/logs/stdio
The error messages were all similar to:
llvm\unittests\CodeGen\GlobalISel\LegalizerInfoTest.cpp(54): error C2398: Element '1': conversion from '' to 'unsigned int' requires a narrowing conversion

llvm-svn: 317578