[CodeGen] Get rid of NDEBUG to ensure structure stability.

[CodeGen] Get rid of NDEBUG to ensure structure stability.

I think it's fine to keep this fields around in terms of overhead,
I wasn't able to measure any substantial regression while running the
test suite, but, in case this causes some regression I'm ready to revert
and work on an alternative solution.
This was tested building with clang/gcc both in Debug and Release mode
and passes the test-suite.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251209 91177308-0d34-0410-b5e6-96231b3b80d8