[SROA] Don't preserve the IR names in release builds.

This is espcially important because the new SROA pass goes to great
lengths to provide helpful names for debugging, and as a consequence
they can become very slow to render.

Good for between 5% and 15% of the SROA runtime on some slow test cases
such as the one in PR15412.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177495 91177308-0d34-0410-b5e6-96231b3b80d8