CodeGen: Start removing implicit conversions to/from list iterators, NFC

Start removing implicit conversions to/from list iterators in CodeGen,
ala r249782 for IR.  A lot more to go after this.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249851 91177308-0d34-0410-b5e6-96231b3b80d8
6 files changed