Options.td: O0 and O4 are not Joined options

(I'm not sure how to test this because the Driver will still accept
 e.g. "-O4foo", but it goes in the catch-all -O option instead of -O4.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187602 91177308-0d34-0410-b5e6-96231b3b80d8