Using Inst->setMetadata(..., NULL) should be safe to remove metadata even when

Using Inst->setMetadata(..., NULL) should be safe to remove metadata even when
there is non of that type to remove. This fixes a crasher in the particular
case where the instruction has metadata but no metadata storage in the context
(this is only possible if the instruction has !dbg but no other metadata info).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147285 91177308-0d34-0410-b5e6-96231b3b80d8