IR: Don't cast the end iterator to Instruction*

IR: Don't cast the end iterator to Instruction*

End iterators are usually sentinels, not actually Instruction* at all.
Stop casting to it just to get an iterator back.

There is likely no observable functionality change here right now
(although this is relying on UB, I doubt it was triggering anything),
but I'll be removing the cast soon.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278346 91177308-0d34-0410-b5e6-96231b3b80d8