Fix unused warning in opt builds.

In these builds, the asserts() are completely compiled out of the code
leaving "End" unused. Directly accessing it, should not have a
performance impact, as it is just a data member.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@182634 91177308-0d34-0410-b5e6-96231b3b80d8