CodeGen: assert an instruction is being inserted with the correct iterator.

When "MBB->Insert(It, ...)" is called, we want It to be pointing inside the
correct basic block. No actual failures at the moment, but it's caused problems
before.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221953 91177308-0d34-0410-b5e6-96231b3b80d8