[CFL-AA] CFL-AA should not assert on an va_arg instruction

The CFL-AA implementation was missing a visit* routine for va_arg instructions,
causing it to assert when run on a function that had one. For now, handle these
in a conservative way.

Fixes PR20954.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219718 91177308-0d34-0410-b5e6-96231b3b80d8