Sema: Don't crash when trying to emit a precedence warning on postinc/decrement.

Post-Inc can occur as a binary call (the infamous dummy int argument), but it's
not really a binary operator.

Fixes PR15628.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@178412 91177308-0d34-0410-b5e6-96231b3b80d8