[LCG] Clean up and make NDEBUG verify calls more rigorous with

[LCG] Clean up and make NDEBUG verify calls more rigorous with
make_scope_exit now that we have that utility.

This makes the code much more clear and readable by isolating the check.
It also makes it easy to go through and make sure all the interesting
update routines have a start and end verify so we don't slowly let the
graph drift into an invalid state.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@280619 91177308-0d34-0410-b5e6-96231b3b80d8