Sink the logic to suppress builtin operator overloads in the presence of

Sink the logic to suppress builtin operator overloads in the presence of
user-defined operator overloads on the same enumeral types to the one place
where it is used.

In theory this removes wasted computation from several paths through this code,
but I'm not aware of a case where it actually matters. This is mostly for
cleanliness.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@121630 91177308-0d34-0410-b5e6-96231b3b80d8