Remove incorrect assert.

If we run into the second preprocessor branch chain, the first branch
chain might have already set the maximum branch count on that level to
something > 0.

Fixes PR17645.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@193153 91177308-0d34-0410-b5e6-96231b3b80d8