Codegen: Do not crash when seeing debug intrinsics

We now skip the debug intrinsics which is a lot better than crashing due to
uncopied metadata references. We should step by step investigate which debug
intrinsics we can copy without trouble.

We still keep the debug location metadata.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@201860 91177308-0d34-0410-b5e6-96231b3b80d8