CodeGen: Avoid getNodePtrUnchecked() where we need a Value, NFC

`ilist_iterator<NodeTy>::getNodePtrUnchecked()` is documented as being
for internal use only, but CodeGenPrepare was using it anyway.  This
code relies on pulling out the `Value*` pointer even after the lifetime
of the iterator is over.  But having this pointer available in
ilist_iterator depends on UB in the first place.

Instead, safely pull out the `Value*` when the iterator is alive and
stop using the internal-only API.

There should be no functionality change here.

llvm-svn: 261493