[polly] [RemoveDI][Polly] Use iterators instead of instruction pointers to SetInsertPoint (PR #135336)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

Consider use the `SetInsertPoint(BasicBlock *, BasicBlock::iterator)` overload. It also works when there is no instruction in the BB.

Things like `->getTerminator()` and `->front()` also only work when there is a least one instruction in the block, but will also fail more obviously if that's not the case. 

https://github.com/llvm/llvm-project/pull/135336