[clang] [llvm] [mlir] [polly] [IR][NFC] Update IRBuilder to use InsertPosition (PR #96497)

[Stephen Tozer via cfe-commits]

SLTozer wrote:

> Looks like using InsertPosition in IRBuilder has some overhead: 

Hm, we're adding an extra pointer chase when we give up passing a basicblock - I think then it makes sense to keep the option to pass BB+It, and do so when the caller already knows the BB (while removing the need to pass the BB around for functions that only need an insert point).

https://github.com/llvm/llvm-project/pull/96497