Always name instructions

We currently need to always name instructions, as the polly test suite currently
matches for certain names. We should improve the test suite at some point.

This fixes 'make check-polly' in NDEBUG builds.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@202855 91177308-0d34-0410-b5e6-96231b3b80d8