Silence MSVC warnings about unsigned wrapping without UB

Silence MSVC warnings about unsigned wrapping without UB

Of course, casting an unsigned value too large for 'int' is UB. So,
write out the ternary. LLVM folds it to ADD anyway.

Fixes the warning from r303693 a different way.

Thanks to Erich Keane for pointing this out!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315406 91177308-0d34-0410-b5e6-96231b3b80d8