Fix unused warning in opt builds.

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