[DebugInfo] Maintain DI when converting GEP to bitcast

When a GEP with all zero indices is converted to bitcast, its DI wasn't
copied over to the newly created instruction. This patch fixes that bug.

Patch by Kareem Ergawy!

Differential Revision: https://reviews.llvm.org/D47347

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@333235 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed