ARM64: add an explicit cast to silence a silly warning

GCC 4.8 complains with:
  warning: enumeral and non-enumeral type in conditional expression

Although this is silly and harmless in this case, add an explicit cast to
silence the warning.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205949 91177308-0d34-0410-b5e6-96231b3b80d8